Object lens, optical pickup, and optical disc device

ABSTRACT

An optical pickup includes: a first emitting unit to emit an optical beam of a first wavelength; a second emitting unit to emit an optical beam of a second wavelength; a third emitting unit to emit an optical beam of a third wavelength; an object lens to condense optical beams emitted from the first through third emitting units onto a signal recording face of an optical disc; and a diffraction unit provided on one face of an optical element or the object lens positioned on the optical path of the optical beams of the first through third wavelengths; wherein the diffraction unit includes a generally circular first diffraction region provided on the innermost perimeter, a ring zone shaped second diffraction region provided on the outer side of the first diffraction region, and a ring zone shaped third diffraction region provided on the outer side of the second diffraction region.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation application of U.S. Ser. No.12/142,351 filed Jun. 19, 2008, which contains subject matter related toJapanese Patent Application JP 2007-197961 filed in the Japanese PatentOffice on Jul. 30, 2007, Japanese Patent Application JP 2008-063383filed in the Japanese Patent Office on Mar. 12, 2008, and JapanesePatent Application JP 2007-303610 filed in the Japanese Patent Office onNov. 22, 2007, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an object lens used with an opticalpickup for recording and/or playing information signals to and/or fromthree different types of optical discs, the optical pickup, and anoptical disc device using the optical pickup.

2. Description of the Related Art

As of recent, there has been proposed, as a next-generation optical discformat, an optical disc capable of high density recording, whereinsignals are recorded and played using an optical beam of blue-violetsemiconductor laser beam having a wavelength around 405 nm (hereafterreferred to as “high density recording optical disc”). This high densityrecording optical disc is being proposed with a structure wherein thecover layer for protecting the signal recording layer is thin, 0.1 mmfor example.

In providing an optical pickup compatible with such high densityrecording optical discs, compatibility with CDs (Compact Discs) using awavelength around 785 nm and DVDs (Digital Versatile Discs) using awavelength around 655 nm, according to the related art, is desirable.That is to say, there is demand for an optical pickup and optical discdevice having compatibility among optical discs of multiple formats withdifferent disc structures and accordingly different laserspecifications.

There has been related art which realizes recoding or playing ofinformation signals to/from three types of optical discs of differentformats, such as that shown in FIG. 60, for example. This arrangementinvolves having two types of object lenses and two types of opticalsystems, one corresponding to DVD and CD, and the other to high densityrecording optical discs, with the object lenses being switched overaccording to the wavelength being used.

An optical pickup 430 shown in FIG. 60 realizes recording and/or playingto and/or from optical discs of different types, by having two types ofobject lenses, object lens 433 and object lens 434. The optical pickup430 has a light source unit 432, such as a laser diode or the like,including an emitting unit for emitting an optical beam of a wavelengtharound 785 nm for optical discs such as CDs and an emitting unit foremitting an optical beam of a wavelength around 655 nm for optical discssuch as DVDs, a light source unit 431, such as a laser diode or thelike, including an emitting unit for emitting an optical beam of awavelength around 405 nm for high density recording optical discs, anobject lens 434 for optical discs such as DVDs and CDs, and an objectlens 433 for high density recording optical discs. The optical pickupalso has collimator lenses 442A and 442B, quarter-wave plates 443A and443B, redirecting mirrors 444A and 444B, beam splitters 437 and 438,gratings 439 and 440, a photosensor 445, a multi-lens 446, and so forth.

An optical beam of a wavelength around 785 nm emitted from the lightsource 432 is transmitted through the beam splitter 437 and beamsplitter 438, and is input to the object lens 434. The object lens 434condenses the beam onto the signal recording face of the optical dischaving a protective layer (cover layer) 1.1 mm thick.

In the same way, the optical beam of a wavelength around 655 nm emittedfrom the light source 432 is input to the object lens 434 via exactlythe same optical path, and is condensed onto the signal recording faceof the optical disc having a protective layer 0.6 mm thick. Return lightof a wavelength of 785 nm and of a wavelength of 655 nm reflected off ofthe signal recording face of the optical disc passes through the beamsplitter 438, and is detected by the photosensor 445 having aphotodetector or the like.

An optical beam of a wavelength around 405 nm emitted from the lightsource 431 is reflected at the beam splitter 437, and is input to theobject lens 433 via the beam splitter 438. The object lens 433 condensesthe beam onto the signal recording face of the optical disc having aprotective layer 0.1 mm thick. Return light of a wavelength of 405 nmreflected off of the signal recording face of the optical disc isdetected at the photosensor 445 via the beam splitter 438.

Thus, the optical pickup shown in FIG. 60 realizes recording and/orplaying of three different types of optical discs by having two types ofobject lenses, the object lens 434 for DVDs and CDs, and the object lens433 for high density recording optical discs, thereby realizingcompatibility between multiple types of optical discs.

SUMMARY OF THE INVENTION

However, optical pickups according to the related art such as describedabove have the following problems. First, each optical disc has adifferent optimal inclination of object lens, and with theabove-described optical pickup, using two object lenses 433 and 434means that the attachment angle of the actuator of the object lenses 433and 434 to lens holders may be unsuitable, resulting in a situationwherein an optimal object lens inclination cannot be realized as to anoptical disc, resulting in deterioration in quality of played signals.Also, with the above-described optical pickup, increase in the number ofparts which need to be placed along the optical path of each of the twooptical systems, such as redirecting mirrors, collimator lenses,quarter-wave plates, and so on, is necessitated due to using the twoobject lenses 433 and 434, causing the problem of increased cost andincreased size of the optical pickup. Further, with the above-describedoptical pickup, there is the need to mount the two object lenses 433 and434 on an object lens driving actuator, resulting in a heavier actuator,of which the sensitivity is thus lowered.

As opposed to this arrangement, there is being studied an optical pickupwherein the above problems are solved and further optical parts aresimplified, by having a single object lens used in common by themultiple types of optical discs and the three types of wavelengths. Abasic principle for providing an object lens corresponding to opticalbeams of the three types of wavelengths is to provide a diffraction unitsuch as a diffraction optical element in the optical path upstream ofthe object lens, thereby inputting the beam into the object lens in thestate of diffusion/convergent light, thereby correcting sphericalaberration occurring due to the combination of usage wavelength and themedia.

However, with the optical pickup being studied according to the relatedart, the structure has involved diffraction units being provided onmultiple faces, or the diffractive face having a spherical face shapediffering from the spherical face of the object lens, or there being aneed to provide a liquid crystal device having a complex configurationin the optical path upstream of the object lens. However, each of theseconfigurations have the lens units, diffraction units, liquid crystaldevices, etc., individually formed and then later assembled, meaningthat a rather high level of precision is necessary for positioning theseand adhering multiple diffraction faces, leading to more andincreasingly troublesome and complicated steps in manufacturing, andproblems of failure to meet the necessary precision.

Also, for example, there has been proposed in Japanese Unexamined PatentApplication Publication No. 2004-265573 an optical pickup wherein adiffraction unit is provided on the entire face, but this has only beensuccessful in realizing compatibility of two wavelengths. In order torealize compatibility of three wavelengths, there is the need toseparately provide an object lens corresponding to the other wavelength,and increase in the number of optical parts, and accordingly increasedcomplication of the arrangement, has been a problem.

There has been realized the need to provide an object lens andcondensing optical device used in an optical pickup realizing recordingand/or playing information signals by condensing optical beams on threetypes of optical discs with different usage wavelengths, using a singleshared object lens, without a complicated configuration, the opticalpickup, and an optical disc device using the optical pickup.

An object lens, according to an embodiment of the present invention,used with an optical pickup configured to irradiate optical beams on atleast a first optical disc, a second optical disc of a different typefrom the first optical disc, and a third optical disc of a differenttype from the first and second optical discs, so as to record and/orplay information signals, with the object lens condensing an opticalbeam of a first wavelength corresponding to the first optical disc, anoptical beam of a second wavelength which is longer than the firstwavelength, corresponding to the second optical disc, and an opticalbeam of a third wavelength which is longer than the second wavelength,corresponding to the third optical disc, onto a signal recording face ofa corresponding optical disc, the object lens including: a diffractionunit provided on the input side face or output side face; wherein thediffraction unit includes a generally circular first diffraction regionprovided on the innermost perimeter, a ring zone shaped seconddiffraction region provided on the outer side of the first diffractionregion, and a ring zone shaped third diffraction region provided on theouter side of the second diffraction region; wherein the firstdiffraction region has a first diffraction structure formed in a ringzone shape and having a predetermined depth, which emits diffractedlight of an order of the optical beam of the first wavelength whichpasses therethrough and is condensed on the signal recording face of thefirst optical disc via the object lens, emits diffracted light of anorder of the optical beam of the second wavelength which passestherethrough and is condensed on the signal recording face of the secondoptical disc via the object lens, and emits diffracted light of an orderof the optical beam of the third wavelength which passes therethroughand is condensed on the signal recording face of the third optical discvia the object lens; and wherein the second diffraction region has asecond diffraction structure which is different from the firstdiffraction structure formed in a ring zone shape and having apredetermined depth, which emits diffracted light of an order of theoptical beam of the first wavelength which passes therethrough and iscondensed on the signal recording face of the first optical disc via theobject lens, emits diffracted light of an order of the optical beam ofthe second wavelength which passes therethrough and is condensed on thesignal recording face of the second optical disc via the object lens,and emits diffracted light such that diffracted light of an order otherthan the order of the optical beam of the third wavelength which passestherethrough and is condensed on the signal recording face of the thirdoptical disc via the object lens is dominant; and wherein the thirddiffraction region has a third diffraction structure which is differentfrom the first and second diffraction structures formed in a ring zoneshape and having a predetermined depth, which emits diffracted light ofan order of the optical beam of the first wavelength which passestherethrough and is condensed on the signal recording face of the firstoptical disc via the object lens, emits diffracted light such thatdiffracted light of an order other than the order of the optical beam ofthe second wavelength which passes therethrough and is condensed on thesignal recording face of the second optical disc via the object lens isdominant, and emits diffracted light such that diffracted light of anorder other than the order of the optical beam of the third wavelengthwhich passes therethrough and is condensed on the signal recording faceof the third optical disc via the object lens is dominant.

An optical pickup according to an embodiment of the present inventionincludes: a first emitting unit configured to emit an optical beam of afirst wavelength corresponding to a first optical disc; a secondemitting unit configured to emit an optical beam of a second wavelengthwhich is longer than the first wavelength, corresponding to a secondoptical disc which is different from the first optical disc; a thirdemitting unit configured to emit an optical beam of a third wavelengthwhich is longer than the second wavelength, corresponding to a thirdoptical disc which is different from the first and second optical discs;and an object lens configured to condense optical beams emitted from thefirst through third emitting units onto a signal recording face of anoptical disc; and a diffraction unit provided on one face of an opticalelement or the object lens positioned on the optical path of the opticalbeams of the first through third wavelengths; wherein the diffractionunit includes a generally circular first diffraction region provided onthe innermost perimeter, a ring zone shaped second diffraction regionprovided on the outer side of the first diffraction region, and a ringzone shaped third diffraction region provided on the outer side of thesecond diffraction region; wherein the first diffraction region has afirst diffraction structure formed in a ring zone shape and having apredetermined depth, which emits diffracted light of an order of theoptical beam of the first wavelength which passes therethrough and iscondensed on the signal recording face of the first optical disc via theobject lens, emits diffracted light of an order of the optical beam ofthe second wavelength which passes therethrough and is condensed on thesignal recording face of the second optical disc via the object lens,and emits diffracted light of an order of the optical beam of the thirdwavelength which passes therethrough and is condensed on the signalrecording face of the third optical disc via the object lens; andwherein the second diffraction region has a second diffraction structurewhich is different from the first diffraction structure formed in a ringzone shape and having a predetermined depth, which emits diffractedlight of an order of the optical beam of the first wavelength whichpasses therethrough and is condensed on the signal recording face of thefirst optical disc via the object lens, emits diffracted light of anorder of the optical beam of the second wavelength which passestherethrough and is condensed on the signal recording face of the secondoptical disc via the object lens, and emits diffracted light such thatdiffracted light of an order other than the order of the optical beam ofthe third wavelength which passes therethrough and is condensed on thesignal recording face of the third optical disc via the object lens isdominant; and wherein the third diffraction region has a thirddiffraction structure which is different from the first and seconddiffraction structures formed in a ring zone shape and having apredetermined depth, which emits diffracted light of an order of theoptical beam of the first wavelength which passes therethrough and iscondensed on the signal recording face of the first optical disc via theobject lens, emits diffracted light such that diffracted light of anorder other than the order of the optical beam of the second wavelengthwhich passes therethrough and is condensed on the signal recording faceof the second optical disc via the object lens is dominant, and emitsdiffracted light such that diffracted light of an order other than theorder of the optical beam of the third wavelength which passestherethrough and is condensed on the signal recording face of the thirdoptical disc via the object lens is dominant.

An optical disc device according to the present invention includes: adriving unit configured to hold and rotationally drive an optical discoptionally selected from at least a first optical disc, a second opticaldisc of a different type from the first optical disc, and a thirdoptical disc of a different type from the first and second opticaldiscs; and an optical pickup configured to selectively irradiatemultiple optical beams of different wavelengths to an optical discrotationally driven by the driving unit, so as to record and/or playinformation signals the optical pickup used with the optical disc devicebeing such as described above.

According to the above configurations, due to a diffraction unitprovided on one face of an optical element disposed on an optical pathbetween an emitting unit emitting optical beams and the signal recordingface of an optical disc, optical beams corresponding to each of thetypes of optical discs having different usage wavelengths can besuitably condensed on the signal recording faces thereof with a singleshared object lens, thereby realizing three-wavelength compatibilitywith a common object lens, and realizing excellent recording and/orplaying of signals to and from each optical disc, without a complicatedstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block circuit diagram illustrating an optical device towhich the present invention has been applied;

FIG. 2 is an optical path diagram illustrating the optical system of anoptical pickup to which the present invention has been applied, as afirst embodiment;

FIGS. 3A and 3B are diagram for describing the functions of adiffraction optical element and object lens configuring the opticalpickup shown in FIG. 2, wherein FIG. 3A is a diagram illustrating anoptical beam in a case of generating +1 order diffracted light of anoptical beam of a first wavelength as to a first optical disc forexample, FIG. 3B is a diagram illustrating an optical beam in a case ofgenerating −1 order diffracted light of an optical beam of a secondwavelength as to a second optical disc for example, and FIG. 3C is adiagram illustrating an optical beam in a case of generating −2 orderdiffracted light of an optical beam of a third wavelength as to a thirdoptical disc for example;

FIG. 4 is a diagram for describing a diffraction optical elementconfiguring the optical pickup shown in FIG. 2, showing a correlatedplan view and cross-sectional view of the diffraction optical element;

FIGS. 5A through 5C are diagrams for describing the configuration of thediffraction unit provided on one face of the diffraction optical elementshown in FIG. 4, wherein FIG. 5A is a cross-sectional view illustratingan example of a first diffraction region provided as an inner ring zoneof the diffraction unit, FIG. 5B is a cross-sectional view illustratingan example of a second diffraction region provided as a middle ring zoneof the diffraction unit, and FIG. 5C is a cross-sectional viewillustrating an example of a third diffraction region provided as anouter ring zone of the diffraction unit;

FIG. 6 is a cross-sectional view illustrating an example wherein ablazed form diffraction structure has been formed, as another example ofthe inner ring zone, middle ring zone, and outer ring zone, configuringthe diffraction unit;

FIGS. 7A through 7C show graphs for calculating the diffractionefficiency of an inner ring zone configuration example 1 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=4, and (k1 i, k2 i, k3 i)=(+1, −1,−2);

FIGS. 8A through 8C show graphs for calculating the diffractionefficiency of an inner ring zone configuration example 2 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=6, and (k1 i, k2 i, k3 i)=(+1, −2,−3);

FIGS. 9A through 9C show graphs for calculating the diffractionefficiency of an inner ring zone configuration example 3 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=5, and (k1 i, k2 i, k3 i)=(+2, −1,−2);

FIGS. 10A through 10C show graphs for calculating the diffractionefficiency of an inner ring zone configuration example 4 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=6, and (k1 i, k2 i, k3 i)=(+2, −2,−3);

FIGS. 11A through 11C show graphs for calculating the diffractionefficiency of a middle ring zone configuration example 1 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=3, and (k1 m, k2 m, k3 m)=(−1, +1,+2);

FIGS. 12A through 12C show shows graphs for calculating the diffractionefficiency of a middle ring zone configuration example 2 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=5, and (k1 m, k2 m, k3 m)=(−1, +2,+3);

FIGS. 13A through 13C show graphs for calculating the diffractionefficiency of a middle ring zone configuration example 3 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=5, and (k1 m, k2 m, k3 m)=(−2, +1,+2);

FIGS. 14A through 14C show shows graphs for calculating the diffractionefficiency of an outer ring zone configuration example 1 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=2, and (k1 o, k2 o, k3 o)=(−1, +1,+2);

FIGS. 15A through 15C show shows graphs for calculating the diffractionefficiency of an outer ring zone configuration example 2 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=5, and (k1 o, k2 o, k3 o)=(+1, −2,−3);

FIGS. 16A through 16C show graphs for calculating the diffractionefficiency of an outer ring zone configuration example 3 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=5, and (k1 o, k2 o, k3 o)=(+2, −1,−2);

FIGS. 17A through 17C show graphs for calculating the diffractionefficiency of an outer ring zone configuration example 4 according tothe first embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=5, and (k1 o, k2 o, k3 o)=(−2, +2,+3);

FIGS. 18A and 18B are diagrams describing an example of a condensingoptical device making up the optical pickup to which the presentinvention has been applied, according to the first embodiment, whereinFIG. 18A is a side view illustrating an example of a condensing opticaldevice configured of a diffraction optical element having a diffractionunit on the incident side thereof and an object lens, and FIG. 18B is aside view illustrating a diffraction optical device according to anexample wherein a diffraction unit is integrally formed on the incidentside face of an object lens;

FIG. 19 is an optical path diagram illustrating another example of theoptical system of an optical pickup to which the present invention hasbeen applied, as a first embodiment;

FIG. 20 is an optical path diagram illustrating the optical system of anoptical pickup to which the present invention has been applied, as asecond embodiment;

FIGS. 21A through 21C are diagrams for describing the functions of thediffraction optical element and object lens configuring the opticalpickup shown in FIG. 20, wherein FIG. 21A is a diagram illustrating anoptical beam in a case of generating +1 order diffracted light of anoptical beam of a first wavelength as to a first optical disc forexample, FIG. 21B is a diagram illustrating an optical beam in a case ofgenerating +1 order diffracted light of an optical beam of a secondwavelength as to a second optical disc for example, and FIG. 21C is adiagram illustrating an optical beam in a case of generating +1 orderdiffracted light of an optical beam of a third wavelength as to a thirdoptical disc for example;

FIG. 22 is a diagram for describing the diffraction optical elementconfiguring the optical pickup shown in FIG. 20, showing a correlated aplan view and cross-sectional view of the diffraction optical element;

FIGS. 23A through 23C are diagrams for describing the configuration ofthe diffraction unit provided on one face of the diffraction opticalelement shown in FIG. 22, wherein FIG. 23A is a cross-sectional viewillustrating an example wherein first through third diffraction regionsprovided as the inner ring zone, middle ring zone, and outer ring zone,of the diffraction unit, respectively, are formed in a blazed form, FIG.23B is a cross-sectional view illustrating another example of the seconddiffraction region provided as the middle ring zone of the diffractionunit, with the second diffraction region formed in a staircase form asanother example, and FIG. 23C is a cross-sectional view illustratinganother example of the third diffraction region provided as the outerring zone of the diffraction unit, with the third diffraction regionformed in a staircase form as another example;

FIG. 24 is a diagram for describing spherical aberration correctionpossibility at the diffraction region of the diffraction unitconfiguring the optical pickup which is used for diffracting the threewavelengths (inner ring zone), showing points plotted according to therelation between wavelength×diffraction order and protective layerthickness, and the design line of the object lens, in a case wherein (k1i, k2 i, k3 i)=(+1, +1, +1);

FIGS. 25A through 25C are diagrams illustrating the longitudinalaberration of effect term ΔWn due to refractive index fluctuation of thecomposition material under change in temperature, the effect term ΔWλdue to wavelength fluctuation, and the sum ΔW of the effect terms ΔWnand ΔWλ, wherein FIG. 25A is a diagram illustrating the longitudinalaberration of each, in a case of selecting a negative diffraction order,FIG. 25B is a diagram illustrating the longitudinal aberration of each,in a case of selecting a positive diffraction order, and FIG. 25C is adiagram illustrating the longitudinal aberration of each, in a case ofselecting a positive diffraction order and also selecting relativelyhigh order diffraction orders for the middle ring zone and outer ringzone;

FIGS. 26A and 26B are diagrams for describing the longitudinalaberration illustrated in FIGS. 25A through 25C, wherein FIG. 26A isdiagram illustrating the state of longitudinal aberration with a lenshaving no aberration, and FIG. 26B is a diagram illustrating a line LBindicating the state of longitudinal aberration with a lens havingaberration;

FIGS. 27A through 27C show graphs for calculating the diffractionefficiency of an example 1 and example 2 of an inner ring zone accordingto the second embodiment, illustrating the change in the diffractionefficiency of the optical beams of each wavelength as to change in thegroove depth d in a case wherein S=∞, and (k1 i, k2 i, k3 i)=(+1, +1,+1);

FIGS. 28A through 28C show graphs for calculating the diffractionefficiency of an example 1 of a middle ring zone according to the secondembodiment, illustrating the change in the diffraction efficiency of theoptical beams of each wavelength as to change in the groove depth d in acase wherein S=3, and (k1 m, k2 m, k3 m)=(+1, +1, +1);

FIGS. 29A through 29C show graphs for calculating the diffractionefficiency of an example 1 of an outer ring zone according to the secondembodiment, illustrating the change in the diffraction efficiency of theoptical beams of each wavelength as to change in the groove depth d in acase wherein S=∞, and (k1 o, k2 o, k3 o)=(+1, +2, +2);

FIG. 30 is a diagram for describing flaring at the example 1 of an outerring zone according to the second embodiment, showing points plottedaccording to the relation between wavelength×diffraction order andprotective layer thickness, and the design line of the object lens, in acase wherein (k1 o, k2 o, k3 o)=(+1, +2, +2);

FIGS. 31A through 31C show graphs for calculating the diffractionefficiency of an example 2 of a middle ring zone according to the secondembodiment, illustrating the change in the diffraction efficiency of theoptical beams of each wavelength as to change in the groove depth d in acase wherein S=∞, and (k1 m, k2 m, k3 m)=(+3, +2, +2);

FIGS. 32A through 32C show for calculating the diffraction efficiency ofan example 2 of an outer ring zone according to the second embodiment,illustrating the change in the diffraction efficiency of the opticalbeams of each wavelength as to change in the groove depth d in a casewherein S=∞, and (k1 o, k2 o, k3 o)=(+4, +3, +3);

FIG. 33 is a diagram for describing flaring at the example 2 of a middlering zone according to the second embodiment, showing points plottedaccording to the relation between wavelength×diffraction order andprotective layer thickness, and the design line of the object lens, in acase wherein (k1 m, k2 m, k3 m)=(+3, +2, +2);

FIG. 34 is a diagram for describing flaring at the example 2 of an outerring zone according to the second embodiment, showing points plottedaccording to the relation between wavelength×diffraction order andprotective layer thickness, and the design line of the object lens, in acase wherein (k1 o, k2 o, k3 o)=(+4, +3, +3);

FIGS. 35A and 35B are diagrams for describing an example of a condensingoptical device making up the optical pickup to which the presentinvention has been applied, according to the second embodiment, whereinFIG. 35A is a side view illustrating a condensing optical deviceconfigured of a diffraction optical element having a diffraction unit onthe incident side thereof, and an object lens, and FIG. 35B is a sideview illustrating a condensing optical device according to an examplewherein a diffraction unit is integrally formed on the incident sideface of the object lens;

FIG. 36 is an optical path diagram illustrating another example of theoptical system of an optical pickup to which the present invention hasbeen applied, as a second embodiment;

FIG. 37 is an optical path diagram illustrating the optical system of anoptical pickup to which the present invention has been applied, as athird embodiment;

FIGS. 38A through 38C are diagrams for describing the functions of thediffraction unit configuring the optical pickup shown in FIG. 37, and isa diagram for describing the functions of a diffraction optical elementprovided with a diffraction unit and having diffraction functions and anobject lens having refractive functions, with reference to an examplewherein the diffraction unit is provided to an optical element separatefrom the object lens, wherein FIG. 38A is a diagram illustrating anoptical beam in a case of generating +1 order diffracted light of anoptical beam of a first wavelength as to a first optical disc forexample, FIG. 38B is a diagram illustrating an optical beam in a case ofgenerating −1 order diffracted light of an optical beam of a secondwavelength as to a second optical disc for example, and FIG. 38C is adiagram illustrating an optical beam in a case of generating −2 orderdiffracted light of an optical beam of a third wavelength as to a thirdoptical disc for example;

FIG. 39 is a diagram for describing the object lens configuring theoptical pickup shown in FIG. 37, showing a correlated plan view andcross-sectional view of the object lens;

FIGS. 40A through 40C are diagrams for describing the configuration ofthe diffraction unit provided on one face of the object lens shown inFIG. 39, wherein FIG. 40A is a cross-sectional view illustrating a shapeas to the reference face as an example of the first diffraction regionprovided as the inner ring zone of the diffraction unit, FIG. 40B is across-sectional view illustrating a shape as to the reference face as anexample of the second diffraction region provided as the middle ringzone of the diffraction unit, and FIG. 40C is a cross-sectional viewillustrating a shape as to the reference face as an example of the thirddiffraction region provided as the outer ring zone of the diffractionunit;

FIG. 41 is a diagram for describing spherical aberration correctionpossibility at the diffraction region of the diffraction unitconfiguring the optical pickup which is used for diffracting the threewavelengths (inner ring zone) with reference to the inner ring zone ofan example 1, showing points plotted according to the relation betweenwavelength×diffraction order and protective layer thickness, and thedesign line of the object lens, in a case wherein (k1 i, k2 i, k3i)=(+1, −1, −2);

FIG. 42 is a diagram conceptually illustrating that spherical aberrationcan be corrected using divergent light, illustrating that the plottedpoints Pλ1, Pλ2′, and Pλ3′ are positioned on a straight line by the plotpositions being shifted due to the second and third wavelengths havingbeen input in a state of divergent rays as compared to the state in FIG.41;

FIG. 43 is a diagram for describing the relation between the diffractionorders k1 and k3 selected at the diffraction unit regarding the firstand third wavelengths, and the focal distance of the object lens as tothe third wavelength, and is a diagram illustrating the change in thefocal distance as to the third wavelength as the diffraction order k3 ofthe third wavelength changes, for each diffraction order k1 of the firstwavelength;

FIGS. 44A through 44C show graphs for calculating the diffractionefficiency of an example 1 of an inner ring zone according to the thirdembodiment, illustrating the change in the diffraction efficiency of theoptical beams of each wavelength as to change in the groove depth d in acase wherein S=4, and (k1 i, k2 i, k3 i)=(+1, −1, −2);

FIGS. 45A through 45C show graphs illustrating change in the diffractionefficiency of a reference example for comparison with the inner ringzone of the example 1 shown in FIG. 44, illustrating the change in thediffraction efficiency of the optical beams of each wavelength as tochange in the groove depth d in a case of a blazed form (S=∞), and (k1i, k2 i, k3 i)=(+1, +1, +1);

FIGS. 46A through 46C are diagrams for describing a technique fordetermining the pitch of the diffraction structure, wherein FIG. 46A isa diagram indicating the design phase amount φ to be provided to thedesign wavelength λ0 at each position in the radial direction, FIG. 46Bis a diagram illustrating indicating the phase amount φ′ to be actuallyprovided at each position in the radial direction based on φ in FIG.46A, and FIG. 46C is a diagram conceptually illustrating the shape ofthe diffraction structure for providing the phase amount φ′ shown inFIG. 46B;

FIG. 47 is a diagram illustrating another example of the middle ringzone configuring the diffraction unit, and is a cross-sectional viewillustrating a shape as to the reference face as an example of thesecond diffraction region where a staircase from diffraction structureis formed;

FIG. 48 is a diagram for describing flaring at the middle ring zone inthe example 1 of the third embodiment, showing points plotted accordingto the relation between wavelength×diffraction order and protectivelayer thickness, and the design line of the object lens, in a casewherein (k1 m, k2 m, k3 m)=(+3, +2, +2);

FIG. 49 is a diagram for describing flaring at the outer ring zone inthe example 1 of the third embodiment, showing points plotted accordingto the relation between wavelength×diffraction order and protectivelayer thickness, and the design line of the object lens, in a casewherein (k1 o, k2 o, k3 o)=(+4, +2, +2);

FIGS. 50A through 50C show graphs for calculating the diffractionefficiency of the example 1 of the middle ring zone according to thethird embodiment, illustrating the change in the diffraction efficiencyof the optical beams of each wavelength as to change in the groove depthd in a case wherein S=∞, and (k1 m, k2 m, k3 m)=(+3, +2, +2);

FIGS. 51A through 51C show graphs for calculating the diffractionefficiency of the example 1 of the outer ring zone according to thethird embodiment, illustrating the change in the diffraction efficiencyof the optical beams of each wavelength as to change in the groove depthd in a case wherein S=∞, and (k1 o, k2 o, k3 o)=(+4, +2, +2);

FIGS. 52A through 52C show graphs for calculating the diffractionefficiency of the example 2 of the inner ring zone according to thethird embodiment, illustrating the change in the diffraction efficiencyof the optical beams of each wavelength as to change in the groove depthd in a case wherein S=3, and (k1 i, k2 i, k3 i)=(0, −1, −2);

FIGS. 53A through 53C show graphs for calculating the diffractionefficiency of the example 2 of the middle ring zone according to thethird embodiment, illustrating the change in the diffraction efficiencyof the optical beams of each wavelength as to change in the groove depthd in a case wherein S=∞, and (k1 m, k2 m, k3 m)=(0, −1, −3);

FIGS. 54A through 54C show graphs for calculating the diffractionefficiency of the example 2 of the outer ring zone according to thethird embodiment, illustrating the change in the diffraction efficiencyof the optical beams of each wavelength as to change in the groove depthd in a case wherein S=∞, and (k1 o, k2 o, k3 o)=(+1, +1, +1);

FIG. 55 is a diagram for describing spherical aberration correctionpossibility at the inner ring zone in the example 2 of the thirdembodiment, showing points plotted according to the relation betweenwavelength×diffraction order and protective layer thickness, and thedesign line of the object lens, in a case wherein (k1 i, k2 i, k3i)=(+0, −1, −2);

FIG. 56 is a diagram for describing flaring at the middle ring zone inthe example 2 of the third embodiment, showing points plotted accordingto the relation between wavelength×diffraction order and protectivelayer thickness, and the design line of the object lens, in a casewherein (k1 m, k2 m, k3 m)=(0, −1, −3);

FIG. 57 is a diagram for describing flaring at the outer ring zone inthe example 2 of the third embodiment, showing points plotted accordingto the relation between wavelength×diffraction order and protectivelayer thickness, and the design line of the object lens, in a casewherein (k1 o, k2 o, k3 o)=(+1, +1, +1);

FIGS. 58A and 58B are diagrams for describing an example of a condensingoptical device making up the optical pickup to which the presentinvention has been applied, according to the third embodiment, whereinFIG. 58A is a side view illustrating a condensing optical device havinga diffraction unit according to an example of being configured of anobject lens with a diffraction unit integrally formed on the incidentside thereof, and FIG. 58B is a side view illustrating a condensingoptical device according to an example configured of a diffractionoptical element having a diffraction unit formed on the incident sidethereof, and an object lens;

FIG. 59 is an optical path diagram illustrating another example of theoptical system of an optical pickup to which the present invention hasbeen applied; and

FIG. 60 is an optical path diagram illustrating an example of an opticalsystem of an optical pickup according to the related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of an optical disc device using an optical pickup to whichthe present invention has been applied will be described with referenceto the drawings.

<1> Overall Configuration of Optical Disc Device FIG. 1

An optical disc device 1 to which the present invention has been appliedincludes, as shown in FIG. 1, an optical pickup 3 for performinginformation recording/playing to and from an optical disc 2, a spindlemotor 4 serving as a driving device for rotationally driving the opticaldisc 2, and a sled motor 5 for moving the optical pickup 3 in the radialdirection of the optical disc 2. The optical disc device 1 is an opticaldisc device realizing compatibility between three standards, wherebyinformation can be recorded and/or played to/from three types of opticaldiscs with different formats, and optical discs with layered recordinglayers. Note that the optical pickup in the optical disc device 1 is notrestricted to the optical pickup 3, and that later-described opticalpickups 103, 203, and so forth, may be used as well.

Optical discs used here include, for example, optical discs usingsemiconductor laser of an emission wavelength around 785 nm, such as CD(Compact Disc), CD-R (Recordable), CD-RW (ReWritable), and so forth,optical discs using semiconductor laser of an emission wavelength around655 nm, such as DVD (Digital Versatile Disc), DVD-R (Recordable), DVD-RW(ReWritable), DVD+RW (ReWritable), and so forth, and further highdensity recording optical discs using a semiconductor laser of a shorteremission wavelength around 405 nm (blue-violet), capable of high densityrecording, such as BD (Blu-ray Disc (a registered trademark)) and soforth.

Hereinafter, the three types of optical discs 2 which the optical discdevice 1 records information to or plays information from will bedescribed as a first optical disc 11 such as BD, described above asbeing capable of high density recording, which has a protective layerformed to a thickness of around 0.1 mm and uses an optical beam of awavelength around 405 nm as the recording/playing beam, a second opticaldisc 12 such as DVD which has a protective layer formed to a thicknessof around 0.6 mm and uses an optical beam of a wavelength around 655 nmas the recording/playing beam, and a third optical disc 13 such as CDwhich has a protective layer formed to a thickness of around 1.1 mm anduses an optical beam of a wavelength around 785 nm as therecording/playing beam.

Driving of the spindle motor 4 and sled motor 5 of the optical discdevice 1 is controlled by a servo control unit 9 controlled based oninstructions from a system controller 7 also serving as a disc typedetermination unit, depending on the type of disc, and are driven at acertain revolution according to the first optical disc 11, secondoptical disc 12, and third optical disc 13, for example.

The optical pickup 3 is an optical pickup having a three wavelengthcompatible optical system, wherein optical beams of differentwavelengths are irradiated onto the recording layers of optical discs ofdifferent standards from the protective layer side, and reflected lightof the optical beams off of the recording layer is detected. The opticalpickup 3 outputs signals corresponding to each of the optical beams,from the detected reflected light.

The optical disc device 1 includes a preamp 14 for generating focuserror signals, tracking error signals, RF signals, and so forth, basedon signals output from the optical pickup 3, a signalmodulator/demodulator and error correction code block (hereinafterreferred to as signal modulator/demodulator & ECC block) 15 fordemodulating signals from the preamp 14 or modulating signals from anexternal computer 17 or the like, an interface 16, a D/A-A/D converter18, and audio-visual processing unit 19, and an audio-visual signalinput/output unit 20.

Based on the output form the photosensor, the preamp 14 generates focuserror signals by the astigmatic method or the like, generates trackingerror signals by the three-beam method, DPD, DPP, or the like, furthergenerates RF signals, and outputs the RF signals to the signalmodulator/demodulator & ECC block 15. Also, the preamp 14 outputs focuserror signals and tracking error signals to the servo control unit 9.

At the time of recording data to the first optical disc 11, the signalmodulator/demodulator & ECC block 15 performs error correctionprocessing according to LDC-ECC and BIS or the like on the digitalsignals input from the interface 16 or D/A-A/D converter 18, and thenperforms modulation such as 1-7PP or the like. At the time of recordingdata to the second optical disc 12, the signal modulator/demodulator &ECC block 15 performs error correction processing such as PC (ProductCode) or the like, and then performs modulation such as 8-16 modulationor the like. At the time of recording data to the third optical disc 13,the signal modulator/demodulator & ECC block 15 performs errorcorrection processing such as CIRC or the like, and then performsmodulation such as 8-14 modulation or the like. The signalmodulator/demodulator & ECC block 15 then outputs the modulated data toa laser control unit 21. Further, when playing each of the opticaldiscs, the signal modulator/demodulator & ECC block 15 performsdemodulation processing based on the RF signals input from the preamp14, and then further performs error correction processing, and outputsthe data to the interface 16 or D/A-A/D converter 18.

For an arrangement wherein data is to be compressed and recorded, acompression/decompression unit may be provided between the signalmodulator/demodulator & ECC block 15 and the interface 16 or D/A-A/Dconverter 18. In this case, the data is compressed with a format such asMPEG2, MPEG4, or the like.

The servo control unit 9 receives input of focus error signals andtracking error signals from the preamp 14. The servo control unit 9generates focus servo signals and tracking servo signals such that thefocus error signals and tracking error signals become zero, and driveand control an object lens driving unit such as a biaxial actuator orthe like driving the object lens, based on the servo signals. Also,synchronization signals or the like are detected from the output fromthe preamp 14, and servo control of the spindle motor is performed byCLV (Constant Linear Velocity), CAV (Constant Angular Velocity), acombination thereof, or the like.

The laser control unit 21 controls the laser source of the opticalpickup 3. Particularly, with this specific example, control is effectedby the laser control unit 21 such that the laser source output powerdiffers between the recording mode and playback mode. Further, controlis effected by the laser control unit 21 such that the laser sourceoutput power differs depending on the type of the optical disc 2. Thelaser control unit 21 switches over the laser source of the opticalpickup 3 depending on the type of optical disc 2 detected by a disc typedetermination unit 22.

The disc type determination unit 22 can detect the different formats ofthe optical disc 2 by detecting change in the amount of reflected lightfrom the first through third optical discs 11, 12, and 13, fromdifference in surface reflectivity, shape and other externaldifferences, and so forth.

Each block making up the optical disc device 1 is configured so as to becapable of signal processing in accordance with the specifications ofthe optical disc 2 which has been mounted, based on the detectionresults at the disc type determination unit 22.

The system controller 7 controls the entire device in accordance withthe type of optical disc 2 determined at the disc type determinationunit 22. Also, the system controller 7 identifies the recording positionor playing position of the optical disc regarding whichrecording/playing is to be performed, based on address information andTOC (Table of Contents) information recorded in premastered bits orgrooves or the like on the innermost portion of the optical disc, andcontrols the components based on the determined position, in accordancewith operation input from the user.

With the optical disc device 1 configured thus, the optical disc 2 isrotationally driven by the spindle motor 4, the sled motor 5 is drivenand controlled in accordance with control signals from the servo controlunit 9, and the optical pickup 3 is moved to a position corresponding tothe desired recording track on the optical disc 2, thereby performingrecording/playing of information to/from the optical disc 2.

Specifically, at the time of performing recording/playing with theoptical disc device 1, the servo control unit 9 rotates the optical disc2 by CAV or CLV or a combination thereof. The optical pickup 3irradiates an optical beam from the light source onto the optical disc 2and detects the returning optical beam therefrom with the photosensor,generates focus error signals and tracking error signals, and performsfocus servo and tracking servo control by driving the object lens withan object lens driving mechanism, based on the focus error signals andtracking error signals.

Also, at the time of recording with the optical disc device 1, signalsfrom an external computer 17 are input to the signalmodulator/demodulator & ECC block 15 via the interface 16. The signalmodulator/demodulator & ECC block 15 adds the above-describedpredetermined error correction code to the digital data input from theinterface 16 or the D/A-A/D converter 18, and after performing furtherpredetermined modulation processing, generates recording signals. Thelaser control unit 21 controls the laser light source of the opticalpickup 3 based on the recording signals generated at the signalmodulator/demodulator & ECC block 15, and records onto a predeterminedoptical disc.

Also, at the time of playing information recorded in an optical disc 2with the optical disc device 1, the signal modulator/demodulator & ECCblock 15 performs demodulation processing on signals detected with thephotosensor. In the event that the recorded signals demodulated by thesignal modulator/demodulator & ECC block 15 are for computer datastorage, these are output to the external computer 17 via the interface16. Accordingly, the external computer 17 can operate based on thesignals recorded on the optical disc 2. Also, in the event that therecorded signals demodulated by the signal modulator/demodulator & ECCblock 15 are for audio-visual, the signals are subjected todigital/analog conversion at the D/A-A/D converter 18, and supplied tothe audio-visual processing unit 19. Audio-visual processing isperformed at the audio-visual processing unit 19, and signals are outputto unshown external speakers or a monitor, via the audio-visual signalinput/output unit 20.

Now, the recording/playing optical pickups 3, 103, 203, etc., used withthe above-described optical disc device 1, will be described in detail.

<2> First Embodiment of Optical Pickup FIGS. 2 through 19

First, an optical pickup 3 to which the present invention is appliedwill be described as a first embodiment of the optical pickup accordingto the present invention, with reference to FIGS. 2 through 19. Asdescribed above, the optical pickup 3 is an optical pickup whichselectively irradiates multiple optical beams with different wavelengthsonto three types of optical discs arbitrarily selected from firstthrough third optical discs 11, 12, and 13, of which the format such asthe thickness of the protective layer differs, thereby performingrecording and/or playing of information signals.

As shown in FIG. 2, the optical pickup 3 to which the present inventionhas been applied includes a first light source 31 having a firstemitting unit for emitting an optical beam of a first wavelength, asecond light source 32 having a second emitting unit for emitting anoptical beam of a second wavelength longer than the first wavelength, athird light source 33 having a third emitting unit for emitting anoptical beam of a third wavelength longer than the second wavelength, anobject lens 34 for condensing optical beams emitted from the emittingunit of the first through third emitting units onto the signal recordingface of an optical disc 2, and a diffraction optical element 35 providedon the optical path between the first through third emitting units andthe object lens 34.

Also, the optical pickup 3 includes a first beam splitter 36 providedbetween the second and third emitting units and the diffraction opticalelement 35, serving as an optical path synthesizing unit forsynthesizing the optical paths of the optical beam of the secondwavelength that has been emitted from the second emitting unit and theoptical beam of the third wavelength that has been emitted from thethird emitting unit, a second beam splitter 37 provided between thefirst beam splitter 36 and the diffraction optical element 35, servingas an optical path synthesizing unit for synthesizing the optical pathof the optical beams of the second and third wavelengths of which theoptical paths have been synthesized by the first beam splitter 36 andthe optical beam of the first wavelength that has been emitted from thefirst emitting unit, and a third beam splitter 38 provided between thesecond beam splitter 37 and the diffraction optical element 35, servingas an optical path splitting unit for splitting the outgoing opticalpath of the optical beams of the first through third wavelengthssynthesized at the second beam splitter 37 from the returning opticalpath of the optical beam of the first through third wavelengthsreflected off of the optical disc (hereinafter also referred to as“return path”).

Further, the optical pickup 3 has a first grating 39 provided betweenthe first emitting unit of the first light source unit 31 and the secondbeam splitter 37, for diffracting the optical beam of the firstwavelength that has been emitted from the first emitting unit into threebeams, for detection of tracking error signals and so forth, a secondgrating 40 provided between the second emitting unit of the second lightsource unit 32 and the first beam splitter 36, for diffracting theoptical beam of the second wavelength that has been emitted from thesecond emitting unit into three beams, for detection of tracking errorsignals and so forth, and a third grating 41 provided between the thirdemitting unit of the third light source unit 33 and the first beamsplitter 36, for diffracting the optical beam of the third wavelengththat has been emitted from the third emitting unit into three beams, fordetection of tracking error signals and so forth.

Also, the optical pickup 3 has a collimator lens 42 provided between thethird beam splitter 38 and the diffraction optical element 35, servingas a divergent angle conversion unit for converting the divergent angleof the optical beams of the first through third wavelengths of which theoptical paths have been synthesized at the third beam splitter 38 so asto be adjusted into a state of generally parallel light or a statediffused or converged as to generally parallel light, and outputting, aquarter-wave plate 43 provided between the collimator lens 42 and thediffraction optical element 35, so as to provide quarter-wave phasedifference to the optical beams of the first through third wavelengthsof which the divergent angle has been adjusted by the collimator lens42, and a redirecting mirror 44 provided between the diffraction opticalelement 35 and the quarter-wave plate 43, for redirecting by reflectionthe optical beam which has passed through the above-described opticalparts within a plane generally orthogonal to the optical axis of theobject lens 34 and diffraction optical element 35, so as to emit theoptical beam in the direction toward the optical axis of the object lens34 and diffraction optical element 35.

Further, the optical pickup 3 includes a photosensor 45 for receivingand detecting the optical beams of the first through third wavelengthssplit at the third beam splitter 38 on the return path from the opticalbeam of the first through third wavelengths on the outgoing path, and amulti lens 46 provided between the third beam splitter 38 and thephotosensor 45, for condensing optical beams of the first through thirdwavelengths on the return path split at the third beam splitter 38 ontothe photoreception face of a photodetector or the like of thephotosensor 45, and also providing astigmatism for detecting focus errorsignals or the like.

The first light source 31 has a first emitting unit for emitting anoptical beam of a first wavelength around 405 nm onto the first opticaldisc 11. The second light source 32 has a second emitting unit foremitting an optical beam of a second wavelength around 655 nm onto thesecond optical disc 12. The third light source 33 has a third emittingunit for emitting an optical beam of a third wavelength around 785 nmonto the third optical disc 13. Note that while the first through thirdemitting units are configured disposed at individual light sources 31,32, and 33, the invention is not restricted to this, and an arrangementmay be made wherein two emitting units of the first through thirdemitting units are disposed at one light source and the remainingemitting unit is disposed at another light source, or wherein the firstthrough third emitting units are disposed so as to form a light sourceat generally the same position.

The object lens 34 condenses the input optical beams of the firstthrough third wavelengths onto the signal recording face of the opticaldisc 2. The object lens 34 is movably held by an object lens drivingmechanism such as an unshown biaxial actuator or the like. The objectlens 34 is driven along two axes, one in the direction toward/away fromthe optical disc 2, and the other in the radial direction of the opticaldisc 2, by being moved by a biaxial actuator or the like based on thetracking error signals and focus error signals generated from the RFsignals of the return light from the optical disc 2 that has beendetected at the photosensor 45. The object lens 34 condenses opticalbeams emitted from the first through third emitting units such that theoptical beams are always focused on the signal recording face of opticaldisc 2, and also causes the focused optical beam to track a recordingtrack formed on the signal recording face of the optical disc 2. Notethat a configuration wherein the later-described diffraction opticalelement 35 is held by a lens holder of the object lens driving mechanismwhere the object lens 34 is held so as to be integral with the objectlens 34 enables the later-described advantages of a diffraction unit 50provided to the diffraction optical element 35 to be suitably manifestedat the time of field shift of the object lens 34 such as movement in thetracking direction.

The diffraction optical element 35 has, as one face thereof for example,a diffraction unit 50 having multiple diffraction regions on theincident side face thereof, with the diffraction unit 50 diffractingeach of the optical beams of the first through third wavelengths passingthrough each of the multiple diffraction regions into predeterminedorders and inputting into the object lens 34, i.e., inputting into theobject lens 34 as optical beams in a diffused state or converged statehaving a predetermined divergent angle, whereby the single object lens34 can be used to perform suitable condensing of the optical beams ofthe first through third wavelengths such that spherical aberration doesnot occur at the signal recording face of the three types of opticaldiscs corresponding to the optical beams of the first through thirdwavelengths. The diffraction optical element 35 serves as a condensationoptical device along with the object lens 34 to appropriately performcondensation such that no spherical aberration occurs at the signalrecording face of the three types of optical discs corresponding to theoptical beams of the three different wavelengths.

The diffraction optical element 35 having the diffraction unit 50performs diffraction of the first wavelength optical beam BB0 which hastransmitted the diffraction unit 50 so as to become +1st orderdiffracted beam BB1 and inputs to the object lens 34, i.e., as anoptical beam in a diffused state having a predetermined divergent angle,thereby appropriately condensing on the signal recording face of thefirst optical disc 11, as shown in FIG. 3A, performs diffraction of thesecond wavelength optical beam BD0 which has transmitted the diffractionunit 50 so as to become −1st order diffracted beam BD1 and inputs to theobject lens 34, i.e., as an optical beam in a converged state having apredetermined divergent angle, thereby appropriately condensing on thesignal recording face of the second optical disc 12, as shown in FIG.3B, and performs diffraction of the third wavelength optical beam BC0which has transmitted the diffraction unit 50 so as to become −2nd orderdiffracted beam BC1 and inputs to the object lens 34, i.e., as a beam ina converged state having a predetermined divergent angle, therebyappropriately condensing on the signal recording face of the thirdoptical disc 13, as shown in FIG. 3C, for example, whereby suitablecondensation can be performed such that no spherical aberration occursat the signal recording face of the three types of optical discs, with asingle object lens 34. While description has been made here with anexample wherein optical beams of the same wavelength are made to bediffracted beams of the same diffraction order at the multiplediffraction regions of the diffraction unit 50, with reference to FIGS.3A through 3C, the diffraction unit 50 configuring the optical pickup 3to which the present invention is applied enables diffraction ordercorresponding to each wavelength to be set for each region as describedlater, so as to further reduce spherical aberration.

Specifically, as shown in FIGS. 4A and 4B, the diffraction unit 50provided at the incident side face of the diffraction optical element 35has a generally-circular first diffraction region 51 provided on theinnermost portion (hereinafter also referred to as “inner ring zone”), aring-shaped second diffraction region 52 provided on the outer side ofthe first diffraction region 51 (hereinafter also referred to as “middlering zone”), and a ring-shaped third diffraction region 53 provided onthe outer side of the second diffraction region 52 (hereinafter alsoreferred to as “outer ring zone”).

The first diffraction region 51 which is an inner ring zone has a firstdiffraction structure formed having a ring shape with a predetermineddepth, and diffracts the optical beam of the first wavelength that istransmitted therethrough such that diffracted light of an order whichcondenses light so as to form an appropriate spot on the signalrecording face of the first optical disc via the object lens 34 isdominant, i.e., such that maximum diffraction efficiency is manifestedregarding diffracted light of other orders.

The first diffraction region 51 diffracts the optical beam of the secondwavelength that is transmitted therethrough such that diffracted lightof an order which condenses light so as to form an appropriate spot onthe signal recording face of the second optical disc via the object lens34 is dominant, i.e., such that maximum diffraction efficiency ismanifested regarding diffracted light of other orders, by way of thefirst diffraction structure.

The first diffraction region 51 diffracts the optical beam of the thirdwavelength that is transmitted therethrough such that diffracted lightof an order which condenses light so as to form an appropriate spot onthe signal recording face of the third optical disc via the object lens34 is dominant, i.e., such that maximum diffraction efficiency ismanifested regarding diffracted light of other orders, by way of thefirst diffraction structure.

Thus, the first diffraction region 51 has a diffraction structure formedwhereby diffracted light of a predetermined order is dominant in theoptical beam of each wavelength, thereby enabling correction andreduction of spherical aberration at the time of optical beams of eachwavelength that have passed through the first diffraction region 51 andbecome diffracted light of a predetermined order being condensed on thesignal recording face of the respective optical discs by the object lens34.

Specifically, as shown in FIGS. 4 and 5A, the first diffraction region51 is formed with the cross-sectional form of ring shapes centered onthe optical axis being formed in a staircase-like form having apredetermined depth (hereinafter also referred to as “groove depth”) dand a predetermined number of steps S (where S is a positive integer),continuing in the radial direction (also referred to as a “multi-stepstaircase form”). Note that the cross-sectional form of the ring shapesin this diffraction structure means the cross-sectional form of therings taken along a plane including the radial direction of the rings,i.e., a plane orthogonal to the tangential direction of the rings. Also,the diffraction structure having the staircase form with a predeterminednumber of steps S is a structure in which a staircase form having firstthrough S steps, each of which have generally the same depth, continuingin the radial direction, which can be rephrased as saying that thestructure has first through S+1'th diffraction faces formed withgenerally the same interval in the optical axis direction. Also, thepredetermined depth d in the diffraction structure means the lengthalong the optical axis between the diffraction face of the S+1'thdiffraction face which is formed at the side of the staircase formclosest to the surface (i.e., the highest step, which is the shallowestposition) and diffraction face of the first diffraction face which isformed at the side of the staircase form closest to the optical element(i.e., the lowest step, which is the deepest position). This holds truefor later described FIGS. 5B and 5C as well.

Note that while a structure has been illustrated in FIGS. 5A through 5Cwherein the steps of each stepped portion of the staircase shape areformed such that the closer to the outer side in the radial direction,the closer to the surface side the steps are formed, but the inventionis not restricted to this arrangement, and an arrangement may be madewherein the steps of each stepped portion of the diffraction structureformed of the inner ring zone, middle ring zone, and outer ring zone,are formed toward the inner side in the radial direction. Specifically,predetermined diffraction angles and diffraction efficiency can beobtained by setting the dominant diffraction order and later-describedgroove width at each diffraction structure, and also a diffused state orconverged state with a desired diffraction angle can be obtained bysetting the formation direction of the staircase form in accordance withwhether the diffraction order is positive or negative. The symbol R_(o)in FIGS. 5A through 5C represents the direction toward the outer side inthe radial direction of the rings, i.e., the direction away from theoptical axis.

In the first diffraction structure and the later-described second andthird diffraction structures formed at the first diffraction region 51,the groove depth d and number of steps S are determined taking intoconsideration the dominant diffraction order and diffraction efficiency.Also, as shown in FIGS. 5A through 5C, the groove width of each step(the radial-direction dimension of each step portion of the staircaseform) is such that the steps are formed with equal width within onestaircase form, while looking at the different staircase forms formedcontinuously in the radial direction, the value of the step width issmaller at staircase forms further away form the optical axis. Note thatthe groove widths are determined based on phase difference obtained atthe diffraction regions formed with the groove widths, such that thespot condensed on the signal recording face of the optical disc isoptimal.

For example, the diffraction structure of the first diffraction region51 is, as shown in FIG. 5A, a diffraction structure having a staircaseportion including first through fourth steps 51 s 1, 51 s 2, 51 s 3, and51 s 4, formed continuously in the radial direction, wherein the numberof steps is 4 (S=4), and the depth of each step is generally the samedepth (d/4), and first through fifth diffraction faces 51 f 1, 51 f 2,51 f 3, 51 f 4, and 51 f 5 formed at the same intervals of d/4 in theoptical axis direction.

Also, while description is made here with regard to the firstdiffraction region 51 having the cross-sectional form of the ringsformed as a diffraction structure with a multi-step staircase form, anydiffraction structure may be used as long as an optical beam of apredetermined order is dominant as to the optical beam of eachwavelength as described above, so a configuration may be used such asshown in FIG. 6, with a diffraction region 51B having a diffractionstructure wherein the cross-sectional form of the rings is formed asblazed diffraction grating having a predetermined depth d, for example.

Also, in a case wherein the first diffraction region 51 diffracts theoptical beam of the first wavelength which is transmitted therethroughsuch that diffracted light of the k1 i'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, diffracts theoptical beam of the second wavelength which is transmitted therethroughsuch that diffracted light of the k2 i'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, and diffracts theoptical beam of the third wavelength which is transmitted therethroughsuch that diffracted light of the k3 i'th order is dominant, none of k1i, k2 i, and k3 i is zero, k1 i and k2 i are of opposite signs (k1 i×k2i<0), and k2 i and k3 i are of the same sign (k2 i×k3 i>0). Note that inthe above case, k1 i and k3 i are of opposite signs.

Now, with the first diffraction region 51, due to the diffraction orderk1 i of the first wavelength at which the diffraction efficiency ismaximum being set to other than zero, coupling at the object lens 34 canbe reduced, the problem of noise due to light source return light can beprevented, and problems such as having to keep the output of the lightsource emission within a suitable range with the related art can beavoided. Also, with the first diffraction region 51, in the event thatthe diffraction orders k2 i and k3 i of the second and third wavelengthsat which the diffraction efficiency is maximum are set to zero, there isno combination wherein the aberration and efficiency are optimal. Inother words, with the first diffraction region 51, due to thediffraction orders k2 i and k3 i being other than zero, a combinationcan be obtained wherein aberration and efficiency can be ensured.

Also, with the first diffraction region 51, due to the relation of thediffraction orders k1 i, k2 i, and k3 i wherein the diffractionefficiency of each wavelength is maximum being of a relation wherein k1i and k2 i are of opposite signs, and k2 i and k3 i are of the samesign, spherical aberration can be further reduced in a case ofcondensing optical beams of each wavelength on the multiple types ofoptical discs with the same object lens 34. This is based on the ideathat since the design center of the protective layer is often set to 0.1to 0.6 in the event of designing an object lens 34 for theabove-described first through third optical discs, spherical aberrationcan be suppressed by inverting the polarity provided to the optical beamof the first wavelength and the polarity provided to the optical beamsof the second and third wavelengths.

Further, with the first diffraction region 51, the diffraction orders k1i, k2 i, and k3 i of each wavelength wherein the diffraction efficiencyis maximum are set such as to conform to one of the following:

(k1 i, k2 i, k3 i)=(+1, −1, −2), (−1, +1, +2), (+1, −2, −3), (−1, +2,+3), (+2, −1, −2), (−2, +1, +2), (+2, −2, −3), or (−2, +2, +3).

Specific examples of the first diffraction region 51 which is the innerring zone will be given below, with specific numerical values of thedepth d and number of steps S, and the diffraction order of diffractedlight of the order that is dominant in the optical beam of eachwavelength, and the diffraction efficiency of the diffracted light ofeach diffraction order is shown in Table 1. Note that Table 1illustrates Inner Ring Zone Configuration Example 1 through Inner RingZone Configuration Example 4 serving as examples of the firstdiffraction region 51, wherein k1 i in Table 1 indicates the diffractionorder where the diffraction efficiency of the optical beam of the firstwavelength is maximum, eff1 illustrates the diffraction efficiency ofthe diffraction order where the diffraction efficiency of the opticalbeam of the first wavelength is maximum, k2 i indicates the diffractionorder where the diffraction efficiency of the optical beam of the secondwavelength is maximum, eff2 illustrates the diffraction efficiency ofthe diffraction order where the diffraction efficiency of the opticalbeam of the second wavelength is maximum, k3 i indicates the diffractionorder where the diffraction efficiency of the optical beam of the thirdwavelength is maximum, eff3 illustrates the diffraction efficiency ofthe diffraction order where the diffraction efficiency of the opticalbeam of the third wavelength is maximum, d indicates the groove depth ofthe first diffraction region 51, i.e., the distance from the lowest stepof the staircase form to the highest step thereof, and S indicates thenumber of steps of the staircase form of the first diffraction region51.

TABLE 1 Inner Ring Zone Diffraction Efficiency, Diffraction Order,Depth, and Number of Steps, for Each Configuration Example k1i eff₁ K2ieff₂ K3i eff₃ d [μm] s Inner Ring Zone 1 0.81 −1 0.62 −2 0.57 3.8 4Configuration Example 1 Inner Ring Zone 1 0.93 −2 0.65 −3 0.52 5.3 6Configuration Example 2 Inner Ring Zone 2 0.67 −1 0.72 −2 0.67 5.1 5Configuration Example 3 Inner Ring Zone 2 0.63 −2 0.64 −3 0.36 5.8 6Configuration Example 4

Now, the Inner Ring Zone Configuration Example 1 shown in Table 1 willbe described. As shown in Table 1, with the Inner Ring ZoneConfiguration Example 1, with the groove depth d=3.8 (μm) and the numberof steps S=4, the diffraction efficiency eff1=0.81 for the firstwavelength optical beam diffraction order k1 i=+1, the diffractionefficiency eff2=0.62 for the second wavelength optical beam diffractionorder k2 i=−1, and the diffraction efficiency eff3=0.57 for the thirdwavelength optical beam diffraction order k3 i=−2. This Inner Ring ZoneConfiguration Example 1 will be described more specifically withreference to FIGS. 7A through 7C. FIG. 7A is a diagram illustrating thechange in diffraction efficiency of the +1 order diffracted light of theoptical beam of the first wavelength in a case wherein the groove depthd is changed in the staircase form with the number of steps S=4, FIG. 7Bis a diagram illustrating the change in diffraction efficiency of the −1order diffracted light of the optical beam of the second wavelength in acase wherein the groove depth d is changed in the staircase form withthe number of steps S=4, and FIG. 7C is a diagram illustrating thechange in diffraction efficiency of the −2 order diffracted light of theoptical beam of the third wavelength in a case wherein the groove depthd is changed in the staircase form with the number of steps S=4. InFIGS. 7A through 7C, the horizontal axis represents the groove depth innm, and the vertical axis represents the diffraction efficiency(intensity of light). As shown in FIG. 7A, at the position of 3800 nm onthe horizontal axis, eff1 is 0.81, eff2 is 0.62 as shown in FIG. 7B, andeff3 is 0.57 as shown in FIG. 7C.

In the same way in Table 1, with the Inner Ring Zone ConfigurationExample 2, with the groove depth d=5.3 (μm) and S=6, the diffractionefficiency eff1, eff2, and eff3 are obtained for the diffraction ordersk1 i, k2 i, and k3 i, as shown in Table 1 and FIGS. 8A through 8C; withthe Inner Ring Zone Configuration Example 3, with the groove depth d=5.1(μm) and S=5, the diffraction efficiency eff1, eff2, and eff3 areobtained for the diffraction orders k1 i, k2 i, and k3 i, as shown inTable 1 and FIGS. 9A through 9C; and with the Inner Ring ZoneConfiguration Example 4 shown in Table 1 as well, with the groove depthd=5.8 (μm) and S=6, the diffraction efficiency eff1, eff2, and eff3 areobtained for the diffraction orders k1 i, k2 i, and k3 i, as shown inTable 1 and FIGS. 10A through 10C.

The second diffraction region 52 which is a middle ring zone has asecond diffraction structure formed which is ring shaped and has apredetermined depth, and which is a different structure from the firstdiffraction structure. The second diffraction region 52 diffracts theoptical beam of the first wavelength that is transmitted therethroughsuch that diffracted light of an order which condenses light so as toform an appropriate spot on the signal recording face of the firstoptical disc via the object lens 34 is dominant, i.e., such that maximumdiffraction efficiency is manifested regarding diffracted light of otherorders.

The second diffraction region 52 diffracts the optical beam of thesecond wavelength that is transmitted therethrough such that diffractedlight of an order which condenses light so as to form an appropriatespot on the signal recording face of the second optical disc via theobject lens 34 is dominant, i.e., such that maximum diffractionefficiency is manifested regarding diffracted light of other orders, byway of the second diffraction structure.

The second diffraction region 52 diffracts the optical beam of the thirdwavelength that is transmitted therethrough such that diffracted lightof orders other than an order which forms an appropriate spot on thesignal recording face of the third optical disc via the object lens 34is dominant, i.e., such that maximum diffraction efficiency ismanifested regarding diffracted light of other orders, by way of thesecond diffraction structure. Note that the second diffraction region 52can sufficiently reduce diffraction efficiency diffracted light of anorder which forms an appropriate spot on the signal recording face ofthe third optical disc via the object lens 34 for the optical beam ofthe third wavelength that is transmitted therethrough, by way of thesecond diffraction structure.

Thus, the second diffraction region 52 has a diffraction structureformed suitable for diffracted light of a predetermined order to bedominant in the optical beam of each wavelength, thereby enablingcorrection and reduction of spherical aberration at the time of opticalbeams of first and second wavelengths that have passed through thesecond diffraction region 52 and become diffracted light of apredetermined order being condensed on the signal recording face of therespective optical discs by the object lens 34.

Also, the second diffraction region 52 is configured so as to functionas described above regarding the optical beams of the first and secondwavelengths, but for the optical beam of the third wavelength such thatdiffracted light of orders other than diffracted light of an order whichis condensed on the signal recording face of the third optical discafter passing through the second diffraction region 52 and the objectlens 34 is dominant, whereby aperture restriction can be applied to theoptical beam of the third wavelength, such that there is very littleeffect even if the optical beam of the third wavelength which has beentransmitted through the second diffraction region 52 is input to theobject lens 34, there is hardly any effect on the signal recording faceof the third optical disc, i.e., markedly reducing the light quantity ofthe optical beam of the third wavelength which is condensed on thesignal recording face after passing through the second diffractionregion 52 and the object lens 34, to around zero.

Now, the above-described first diffraction region 51 is formed of a sizesuch that the optical beam of the third wavelength which has beentransmitted through the region thereof is input to the object lens 34 inthe same state as an optical beam which has been subjected to aperturerestriction at around NA=0.45, and since the second diffraction region52 formed on the outer side of the first diffraction region 51 does notallow condensation of the optical beam of the third wavelength which hasbeen transmitted through this region on the third optical disc via theoptical lens 34, the diffraction unit 50 which has the first and seconddiffraction regions 51 and 52 configured thus functions so as torestrict the numerical aperture of the optical beam of the thirdwavelength to around NA=0.45. It should be noted however, that while inthis arrangement of the diffraction unit 50, the optical beam of thethird wavelength is subjected to aperture restriction around NA=0.45,but the present invention is not restricted to this, i.e., numericalaperture restriction due to the above configuration is not limited tothis.

Specifically, as shown in FIGS. 4 and 5B, in the same way as with theabove-described first diffraction region 51, the second diffractionregion 52 is formed with the cross-sectional form of ring shapescentered on the optical axis being formed in a staircase-like shapehaving a predetermined depth d and a predetermined number of steps S,continuing in the radial direction in a staircase form. Note that thevalues of the second diffraction region 52 for d and/or S differ fromthose with the first diffraction region 51, so the second diffractionregion 52 has formed a second diffraction structure which differs fromthe diffraction structure formed with the first diffraction region 51.For example, the diffraction structure of the second diffraction region52 is, as shown in FIG. 5B, a diffraction structure having a staircaseportion including first through third steps 52 s 1, 52 s 2, and 52 s 3,formed continuously in the radial direction, wherein the number of stepsis 3 (S=3), and the depth of each step is generally the same depth(d/3), and first through fourth diffraction faces 52 f 1, 52 f 2, 52 f3, and 52 f 4 formed at the same intervals of d/3 in the optical axisdirection.

Also, while description is made here with regard to the seconddiffraction region 52 having the cross-sectional form of the ringsformed as a diffraction structure with a multi-step staircase form, anydiffraction structure may be used as long as an optical beam of apredetermined order is dominant as to the optical beam of eachwavelength as described above, in the same way as with the firstdiffraction region 51, so a configuration may be used such as shown inFIG. 6, with a diffraction region 52B having a diffraction structurewherein the cross-sectional form of the rings is formed as blazeddiffraction grating having a predetermined depth d, for example.

Also, in a case wherein the second diffraction region 52 diffracts theoptical beam of the first wavelength which is transmitted therethroughsuch that diffracted light of the k1 m'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, and diffracts theoptical beam of the second wavelength which is transmitted therethroughsuch that diffracted light of the k2 m'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, the diffractionorders k1 m and k2 m are set such as to conform to one of the following:

(k1 m, k2 m)=(+1, −1), (−1, +1), (+1, −2), (−1, +2), (+2, −1), or (−2,+1).

Specific examples of the second diffraction region 52 which is themiddle ring zone will be given below, with specific numerical values ofthe depth d and number of steps S, and the diffraction order ofdiffracted light of the order that is dominant in the optical beam ofeach wavelength, and the diffraction efficiency of the diffracted lightof each diffraction order is shown in Table 2. Note that Table 2illustrates Middle Ring Zone Configuration Example 1 through Middle RingZone Configuration Example 3, wherein k1 m in Table 2 indicates thediffraction order where the diffraction efficiency of the optical beamof the first wavelength is maximum, eff1 illustrates the diffractionefficiency of the diffraction order where the diffraction efficiency ofthe optical beam of the first wavelength is maximum, k2 m indicates thediffraction order where the diffraction efficiency of the optical beamof the second wavelength is maximum, eff2 illustrates the diffractionefficiency of the diffraction order where the diffraction efficiency ofthe optical beam of the second wavelength is maximum, k3 m indicates thediffraction order where the optical beam of the third wavelength isselected as described below, eff3 illustrates the diffraction efficiencyof the diffraction order where the optical beam of the third wavelengthis selected, d indicates the groove depth of the second diffractionregion 52, i.e., the distance from the lowest step of the staircase formto the highest step thereof, and S indicates the number of steps of thestaircase form of the second diffraction region 52. Note that theasterisks in Table 2 indicate diffraction order for condensing anoptical beam passing through the middle ring zone in this configurationexample so as to appropriately form a spot on the signal recording faceof the corresponding optical disk via the object lens 34, i.e., adiffraction order whereby spherical aberration on the signal recordingface of the corresponding optical disc can be corrected, and “≈0”indicates that the diffraction efficiency is at a state of approximatelyzero.

TABLE 2 Middle Ring Zone Diffraction Efficiency, Diffraction Order,Depth, and Number of Steps, for Each Configuration Example k1m eff₁ K2meff₂ K3m eff₃ d [μm] s Middle Ring −1 0.76 1 0.77 ※ ~0 8.6 3 ZoneConfiguration Example 1 Middle Ring −1 0.91 2 0.54 ※ ~0 14.8 5 ZoneConfiguration Example 2 Middle Ring −2 0.67 1 0.89 ※ ~0 14.1 5 ZoneConfiguration Example 3 * indicates diffraction orders regarding whichspherical aberration is possible

Now, the Middle Ring Zone Configuration Example 1 shown in Table 2 willbe described. As shown in Table 2, with the Middle Ring ZoneConfiguration Example 1, with the groove depth d=8.6 (μm) and the numberof steps S=3, the diffraction efficiency eff1=0.76 for the firstwavelength optical beam diffraction order k1 m=−1, the diffractionefficiency eff2=0.77 for the second wavelength optical beam diffractionorder k2 m=+1. Also, the diffraction efficiency eff3 is approximately 0for the diffraction order k3 m, where optical beams of the thirdwavelength passing through this region are condensed on the signalrecording face of the third optical disc so as to form a spot, via theobject lens 34.

This Middle Ring Zone Configuration Example 1 will be described morespecifically with reference to FIGS. 11A through 11C. FIG. 11A is adiagram illustrating the change in diffraction efficiency of the −1order diffracted light of the optical beam of the first wavelength in acase wherein the depth d is changed in the staircase form with thenumber of steps S=3, FIG. 11B is a diagram illustrating the change indiffraction efficiency of the +1 order diffracted light of the opticalbeam of the second wavelength in a case wherein the depth d is changedin the staircase form with the number of steps S=3, and FIG. 11C is adiagram illustrating the change in diffraction efficiency of the +2order diffracted light of the optical beam of the third wavelength in acase wherein the depth d is changed in the staircase form with thenumber of steps S=3. In FIGS. 11A through 11C, the horizontal axisrepresents the groove depth in nm, and the vertical axis represents thediffraction efficiency (intensity of light). As shown in FIG. 11A, atthe position of 8600 nm on the horizontal axis, eff1 is 0.76, eff2 is0.77 as shown in FIG. 11B, and eff3 is approximately zero as shown inFIG. 11C. The diffraction order k3 m of the optical beam of the thirdwavelength noted by the asterisk in Table 2 is k3 m=+2.

In the same way in Table 2, with the Middle Ring Zone ConfigurationExample 2, with the groove depth d=14.8 (μm) and S=5, the diffractionefficiency eff1, eff2, and eff3 are obtained for the diffraction ordersk1 m, k2 m, and k3 m, as shown in Table 2 and FIGS. 12A through 12C; andwith the Middle Ring Zone Configuration Example 3 shown in Table 2, withthe groove depth d=14.1 (μm) and S=5, the diffraction efficiency eff1,eff2, and eff3 are obtained for the diffraction orders k1 m, k2 m, andk3 m, as shown in Table 2 and FIGS. 13A through 13C.

The third diffraction region 53 which is an outer ring zone has a thirddiffraction structure formed which is ring shaped and has apredetermined depth, and which is a different structure from the firstand second diffraction structures. The third diffraction region 53diffracts the optical beam of the first wavelength that is transmittedtherethrough such that diffracted light of an order which forms anappropriate spot condensed on the signal recording face of the firstoptical disc via the object lens 34 is dominant, i.e., such that maximumdiffraction efficiency is manifested regarding diffracted light of otherorders.

The third diffraction region 53 diffracts the optical beam of the secondwavelength that is transmitted therethrough such that diffracted lightof orders other than an order which forms an appropriate spot condensedon the signal recording face of the second optical disc via the objectlens 34 is dominant, i.e., such that maximum diffraction efficiency ismanifested regarding diffracted light of other orders, by way of thethird diffraction structure. Note that the third diffraction region 53can sufficiently reduce diffraction efficiency diffracted light of anorder which forms an appropriate spot condensed on the signal recordingface of the second optical disc via the object lens 34 for the opticalbeam of the second wavelength that is transmitted therethrough, by wayof the third diffraction structure.

The third diffraction region 53 diffracts the optical beam of the thirdwavelength that is transmitted therethrough such that diffracted lightof orders other than an order which forms an appropriate spot condensedon the signal recording face of the third optical disc via the objectlens 34 is dominant, i.e., such that maximum diffraction efficiency ismanifested regarding diffracted light of other orders, by way of thethird diffraction structure. Note that the third diffraction region 53can sufficiently reduce diffraction efficiency diffracted light of anorder which forms an appropriate spot condensed on the signal recordingface of the third optical disc via the object lens 34 for the opticalbeam of the third wavelength that is transmitted therethrough, by way ofthe third diffraction structure.

Thus, the third diffraction region 53 has a diffraction structuresuitably formed whereby diffracted light of a predetermined order isdominant in the optical beam of each wavelength, thereby enablingcorrection and reduction of spherical aberration at the time of opticalbeams of the first wavelength that has passed through the thirddiffraction region 53 and become diffracted light of a predeterminedorder being condensed on the signal recording face of the respectiveoptical discs by the object lens 34.

Also, the third diffraction region 53 is configured so as to function asdescribed above regarding the optical beams of the first wavelength, butsuch that regarding the second and third wavelength beams, diffractedlight of orders other than diffracted light of an order which iscondensed on the signal recording face of the second and third opticaldiscs after passing through the third diffraction region 53 and theobject lens 34 is dominant, whereby aperture restriction can be appliedto the optical beam of the second wavelength, such even if the opticalbeams of the second and third wavelengths which have been transmittedthrough the third diffraction region 53 are input to the object lens 34,that there is very little effect on the signal recording face of thesecond and third optical discs, i.e., markedly reducing the lightquantity of the optical beams of the second and third wavelengths whichare condensed on the signal recording faces after passing through thethird diffraction region 53 and the object lens 34, to around zero.Also, the third diffraction region 53 can function to subject theoptical beam of the third wavelength to aperture restriction along withthe above-described second diffraction region 52.

Now, the above-described second diffraction region 52 is formed of asize such that the optical beam of the second wavelength which has beentransmitted through the region thereof is input to the object lens 34 inthe same state as an optical beam which has been subjected to aperturerestriction at around NA=0.6, and since the third diffraction region 53formed on the outer side of the second diffraction region 52 does notallow condensation of the optical beam of the second wavelength whichhas been transmitted through this region on the optical disc via theobject lens 34, the diffraction unit 50 which has the second and thirddiffraction regions 52 and 53 configured thus functions so as torestrict the numerical aperture of the optical beam of the secondwavelength to around NA=0.6. It should be noted however, that while inthis arrangement of the diffraction unit 50, the optical beam of thesecond wavelength is subjected to aperture restriction around NA=0.6,but the present invention is not restricted to this, i.e., numericalaperture restriction due to the above configuration is not limited tothis.

Also, the third diffraction region 53 is formed of a size such that theoptical beam of the first wavelength which has been transmitted throughthe region thereof is input to the object lens 34 in the same state asan optical beam which has been subjected to aperture restriction ataround NA=0.85, and since there is no diffraction structure formed onthe outer side of the third diffraction region 53, this does not allowcondensation of the optical beam of the first wavelength which has beentransmitted through this region on the first optical disc via the objectlens 34, and the diffraction unit 50 which has the third diffractionregion 53 configured thus functions so as to restrict the numericalaperture of the optical beam of the first wavelength to around NA=0.85.Note that with the first wavelength optical beam transmitted through thethird diffraction region 53, light of diffraction orders of −1, +1, +2,and −2 is dominant, so the zero-order light transmitted through theregion outside the third diffraction region 53 almost never passesthrough the object lens 34 to be condensed on the first optical disc,but in cases wherein this zero-order does pass through the object lens34 and is condensed on the first optical disc, a configuration may beprovided to perform aperture restriction by providing, at the regionoutside of the third diffraction region 53, either a shielding portionfor shielding optical beams passing through, or a diffraction regionhaving a diffraction structure wherein optical beams of orders otherthan the order of the optical beam passing through the object lens 34 tobe condensed on the first optical disc are dominant. It should be notedhowever, that while in this arrangement of the diffraction unit 50, theoptical beam of the first wavelength is subjected to aperturerestriction around NA=0.85, but the present invention is not restrictedto this, i.e., numerical aperture restriction due to the aboveconfiguration is not limited to this.

Specifically, as shown in FIGS. 4 and 5C, in the same way as with theabove-described first diffraction region 51, the third diffractionregion 53 is formed with the cross-sectional form of ring shapescentered on the optical axis being formed in a staircase-like shapehaving a predetermined depth d and a predetermined number of steps S,continuing in the radial direction in a staircase form. Note that thevalues of the third diffraction region 53 for d and/or S differ fromthose with the first and second diffraction regions 51 and 52, so thethird diffraction region 53 has formed a third diffraction structurewhich differs from the first and second diffraction structures formedwith the first and second diffraction regions 51 and 52. For example,the diffraction structure of the third diffraction region 53 is, asshown in FIG. 5C, a diffraction structure having a staircase portionincluding first and second steps 53 s 1 and 53 s 2, formed continuouslyin the radial direction, wherein the number of steps is 2 (S=2), and thedepth of each step is generally the same depth (d/2), and first throughthird diffraction faces 53 f 1, 53 f 2, and 53 f 3 formed at the sameintervals of d/2 in the optical axis direction.

Also, while description is made here with regard to the thirddiffraction region 53 having the cross-sectional form of the ringsformed as a diffraction structure with a multi-step staircase form, anydiffraction structure may be used as long as an optical beam of apredetermined order is dominant as to the optical beam of eachwavelength as described above, in the same way as with the first andsecond diffraction regions 51 and 52, so a configuration may be usedsuch as shown in FIG. 6, with a diffraction region 53B having adiffraction structure wherein the cross-sectional form of the rings isformed as a blazed form having a predetermined depth d, for example.

Specific examples of the third diffraction region 53 which is the outerring zone will be given below, with specific numerical values of thedepth d and number of steps S, and the diffraction order of diffractedlight of the order that is dominant in the optical beam of eachwavelength, and the diffraction efficiency of the diffracted light ofeach diffraction order is shown in Table 3. Note that Table 3illustrates Outer Ring Zone Configuration Example 1 through Outer RingZone Configuration Example 4, wherein k1 o in Table 3 indicates thediffraction order where the diffraction efficiency of the optical beamof the first wavelength is maximum, eff1 illustrates the diffractionefficiency of the diffraction order where the diffraction efficiency ofthe optical beam of the first wavelength is maximum, k2 o indicates thediffraction order where the optical beam of the second wavelength isselected as described below, eff2 illustrates the diffraction efficiencyof the diffraction order where the optical beam of the second wavelengthis selected, k3 o indicates the diffraction order where the optical beamof the third wavelength is selected as described below, eff3 illustratesthe diffraction efficiency of the diffraction order where the opticalbeam of the third wavelength is selected, d indicates the groove depthof the third diffraction region 53, i.e., the distance from the loweststep of the staircase form to the highest step thereof, and S indicatesthe number of steps of the staircase form of the third diffractionregion 53. Note that the asterisks in Table 3 indicate diffraction orderfor condensing an optical beam passing through the outer ring zone inthis configuration example so as to appropriately form a spot on thesignal recording face of the corresponding optical disk via the objectlens 34, i.e., a diffraction order whereby spherical aberration on thesignal recording face of the corresponding optical disc can becorrected, and “≈0” indicates that the diffraction efficiency is at astate of approximately zero.

TABLE 3 Outer Ring Zone Diffraction Efficiency, Diffraction Order,Depth, and Number of Steps, for Each Configuration Example k1o eff₁ K2oeff₂ K3o eff₃ d [μm] s Outer Ring −1 0.63 ※ ~0 ※ ~0 4.2 2 ZoneConfiguration Example 1 Outer Ring 1 0.78 ※ ~0 ※ ~0 0.5 5 ZoneConfiguration Example 2 Outer Ring 2 0.65 ※ ~0 ※ ~0 1.2 5 ZoneConfiguration Example 3 Outer Ring −2 0.68 ※ ~0 ※ ~0 6.4 5 ZoneConfiguration Example 4 * indicates diffraction orders regarding whichspherical aberration is possible

Now, the Outer Ring Zone Configuration Example 1 shown in Table 3 willbe described. As shown in Table 3, with the Outer Ring ZoneConfiguration Example 1, with the groove depth d=4.2 (μm) and the numberof steps S=2, the diffraction efficiency eff1=0.63 for the firstwavelength optical beam diffraction order k1 o=−1. Also, the diffractionefficiency eff2 is approximately 0 for the second wavelength opticalbeam diffraction order k2 o, where optical beams of the secondwavelength passing through this region are condensed on the signalrecording face of the second optical disc so as to form a spot, via theobject lens 34. Further, the diffraction efficiency eff3 isapproximately 0 for the third wavelength optical beam diffraction orderk3 o, where optical beams of the third wavelength passing through thisregion are condensed on the signal recording face of the third opticaldisc so as to form a spot, via the object lens 34.

Next, this Outer Ring Zone Configuration Example 1 will be describedmore specifically with reference to FIGS. 14A through 14C. FIG. 14A is adiagram illustrating the change in diffraction efficiency of the −1order diffracted light of the optical beam of the first wavelength in acase wherein the depth d is changed in the staircase form with thenumber of steps S=2, FIG. 14B is a diagram illustrating the change indiffraction efficiency of the +1 order diffracted light of the opticalbeam of the second wavelength in a case wherein the depth d is changedin the staircase form with the number of steps S=2, and FIG. 14C is adiagram illustrating the change in diffraction efficiency of the +2order diffracted light of the optical beam of the third wavelength in acase wherein the depth d is changed in the staircase form with thenumber of steps S=2. In FIGS. 14A through 14C, the horizontal axisrepresents the groove depth in nm, and the vertical axis represents thediffraction efficiency (intensity of light). As shown in FIG. 14A, atthe position of 4200 nm on the horizontal axis, eff1 is 0.63, eff2 isapproximately zero as shown in FIG. 14B, and eff3 is approximately zeroas shown in FIG. 14C. The diffraction orders k2 o and k3 o noted by theasterisks in Table 3 are k2 o=+1 and k3 o=+2.

In the same way with the Outer Ring Zone Configuration Example 2 inTable 3, with the groove depth d=0.5 (μm) and S=5, the diffractionefficiency eff1, eff2, and eff3 are obtained for the diffraction ordersk1 o, k2 o, and k3 o, as shown in Table 3 and FIGS. 15A through 15C;with the Outer Ring Zone Configuration Example 3 in Table 3, with thegroove depth d=1.2 (μm) and S=5, the diffraction efficiency eff1, eff2,and eff3 are obtained for the diffraction orders k1 o, k2 o, and k3 o,as shown in Table 3 and FIGS. 16A through 16C; and with the Outer RingZone Configuration Example 4 in Table 3, with the groove depth d=6.4(μm) and S=5, the diffraction efficiency eff1, eff2, and eff3 areobtained for the diffraction orders k1 o, k2 o, and k3 o, as shown inTable 3 and FIGS. 17A through 17C.

The diffraction unit 50, having the first through third diffractionregions 51, 52, and 53 with the configuration such as described above,is capable of condensation of the optical beams of the first throughthird wavelengths passing through the first diffraction region 51 so asto form a suitable spot on the signal recording face of thecorresponding optical disc by being input to the object lens 34, in adivergent angle state wherein no spherical aberration occurs at thesignal recording face of respectively corresponding optical discs viathe common object lens 34, i.e., in a dispersed state or converged statewherein spherical aberration is corrected via the object lens 34, and iscapable of condensation of the optical beams of the first and secondwavelengths passing through the second diffraction region 52 so as toform a suitable spot on the signal recording face of the correspondingoptical disc by being input to the object lens 34, in a divergent anglestate wherein no spherical aberration occurs at the signal recordingface of respectively corresponding optical discs via the common objectlens 34, i.e., in a dispersed state or converged state wherein sphericalaberration is corrected via the object lens 34, and also is capable ofcondensation of the optical beam of the first wavelength passing throughthe third diffraction region 53 so as to form a suitable spot on thesignal recording face of the corresponding optical disc by being inputto the object lens 34, in a divergent angle state wherein no sphericalaberration occurs at the signal recording face of the correspondingoptical disc via the object lens 34, i.e., in a dispersed state orconverged state wherein spherical aberration is corrected via the objectlens 34.

That is to say, the diffraction unit 50 provided on one face of thediffraction optical element 35 disposed on the optical path between thefirst through third emitting units of the optical pickup 3 and thesignal recording face allows optical beams of respective wavelengthspassing through the respective regions (first through third diffractionregions 51, 52, and 53) to be input to the object lens 34 in a statewherein spherical aberration occurring at the signal recording face tobe reduced, so spherical aberration occurring at the signal recordingface when condensing optical beams of the first through thirdwavelengths on the signal recording face of the respective correspondingoptical discs using the common object lens 34 in the optical pickup 3can be minimized, which is to say that three-wavelength compatibility ofthe optical pickup using three types of wavelengths for three types ofoptical discs and a common object lens 34 can be realized, whereininformation signals can be recorded to and/or played from respectiveoptical discs.

Also, the diffraction unit 50 having the first through third diffractionregions 51, 52, and 53 performs diffraction of the optical beam of thethird wavelength passing through the second and third diffractionregions 52 and 53 such that an order other than the diffraction orderwhere the optical beam is appropriately condensed on the signalrecording face of the corresponding type of optical disc via the objectlens 34 is dominant, whereby, with regard to the optical beam of thethird wavelength, only the optical beam portion which has passed throughthe first diffraction region 51 is condensed on the signal recordingface of the optical disc via the object lens 34, and also, the firstdiffraction region 51 is formed to a size which is the predeterminednumerical aperture of the third wavelength optical beam passing throughthis region, whereby aperture restriction can be performed regarding theoptical beam of the third wavelength such that NA= around 0.45, forexample.

Also, the diffraction unit 50 performs diffraction of the optical beamof the second wavelength passing through the third diffraction region 53such that an order other than the diffraction order where the opticalbeam is appropriately condensed on the signal recording face of thecorresponding type of optical disc via the object lens 34 is dominant,whereby, with regard to the optical beam of the second wavelength, onlythe optical beam portion which has passed through the first and seconddiffraction regions 51 and 52 is condensed on the signal recording faceof the optical disc via the object lens 34, and also, the first andsecond diffraction regions 51 and 52 are formed to a size which is thepredetermined numerical aperture of the second wavelength optical beampassing through this region, whereby aperture restriction can beperformed regarding the optical beam of the second wavelength such thatNA= around 0.60, for example.

Also, the diffraction unit 50 places the optical beam of the firstwavelength passing outside of the third diffraction region 53 in a stateso as to not be suitably condensed on the signal recording face of thecorresponding type of optical disc via the object lens 34, or shieldsthe optical beam of the first wavelength passing outside of the thirddiffraction region 53, whereby, with regard to the optical beam of thefirst wavelength, only the optical beam portion which has passed throughthe first through third diffraction regions 51, 52, and 53 is condensedon the signal recording face of the optical disc via the object lens 34,and also, the first through third diffraction regions 51, 52, and 53 areformed to a size which is the predetermined numerical aperture of thefirst wavelength optical beam passing through this region, wherebyaperture restriction can be performed regarding the optical beam of thefirst wavelength such that NA= around 0.85, for example.

Thus, the diffraction unit 50 provided on one face of the diffractionoptical element 35 disposed on the optical path as described above notonly realizes three-wavelength compatibility, but also enables opticalbeams of each wavelength to be input to the common object lens 34 in astate wherein aperture restriction is performed appropriately for eachof the three types of optical discs and optical beams of the firstthrough third wavelengths. That is to say, the diffraction unit 50 notonly has functions of aberration correction corresponding to threewavelengths, but also has functions as an aperture restricting unit.

It should be noted that a diffraction unit can be configured by suitablycombining the above-described diffraction region examples. That is tosay, the diffraction order of each wavelength passing through eachdiffraction region can be selected as appropriate. In the event ofchanging the diffraction order of each wavelength passing through eachdiffraction region, an object lens 34 corresponding to each diffractionorder of each wavelength passing through each diffraction region can beused.

Also, while the first through third diffraction regions 51, 52, and 53have been shown here having a so-called multi-step form diffractionstructure with a staircase form having steps of a predetermined depth, aconfiguration may be used such as shown in FIG. 6, formed as a blazedform. Particularly, with a diffraction region having a diffractionstructure with a shallow groove depth d formed, such as the thirddiffraction region, the manufacturing processes is simplified by formingas a blazed form, thereby simplifying and reducing costs ofmanufacturing.

Also, while description has been made above with the diffraction unit 50configured of the three diffraction regions 51, 52, and 53 formed on theincident side face of the diffraction optical element 35 providedseparately from the object lens 34, as shown in FIG. 18A, the presentinvention is not restricted to this arrangement, and may be provided tothe output side face of the diffraction optical element 35. Further, thediffraction unit 50 having the first through third diffraction regions51, 52, and 53, can be integrally configured on the input or output sideof the object lens 34, or further, as shown in FIG. 18B for example, anobject lens 34B having the diffraction unit 50 on the incident sidethereof may be configured. In the event of providing the diffractionunit 50 on the incident side face of the object lens 34B for example,the planar shape of the above-described diffraction structure iscombined with a reference face at the incident side required for thelens to be able to function as an object lens. While the above-describeddiffraction optical element 35 and the object lens 34 are two separateelements serving as a condensing optical device, the object lens 34Bthus configured functions as a condensing optical device which canperform suitable light condensing such that spherical aberration doesnot occur at the signal recording face of optical discs corresponding toeach of the three optical beams of different wavelengths, with a singleelement. Providing the diffraction unit 50 integrally with the objectlens 34B enables further reduction in optical parts and also reductionin configuration size.

The object lens 34B having a diffraction unit having functions the sameas the diffraction unit 50 provided integrally at the input side oroutput side face realizes three-wavelength compatibility of the opticalpickup 3 by reducing aberration and so forth when used in an opticalpickup, and also reduces the number of parts so as to enablesimplification and reduction in size of the configuration, therebyrealizing high production and reduced costs. Note that theabove-described diffraction unit 50 sufficiently manifests theadvantages thereof with the diffraction structure for aberrationcorrection to realize three-wavelength compatibility being provided on asingle face that has been difficult with the related art, which enablessuch a diffraction element to be integrally formed with the object lens34, further enabling directly forming a diffraction face on a plasticlens, and forming the object lens 34B with which the diffraction unit 50has been integrated of a plastic material further realized improvedproduction and lower costs.

The collimator lens 42 provided between the diffraction optical element35 and the third beam splitter 38 converts the divergent angle of thefirst through third wavelength optical beams of which the optical pathshave been synthesized at the second beam splitter 37 and passed throughthe third beam splitter, and outputs to the quarter-wave plate 43 anddiffraction optical element 35 side, in a generally parallel lightstate, for example. The arrangement wherein the collimator lens 42inputs the optical beams of the first and second wavelengths into theabove-described diffraction optical element 35 with the divergent anglethereof in the state of generally parallel light, and also inputs theoptical beam of the third wavelength into the diffraction opticalelement 35 in a divergent angle state which is slightly diffused orconverged as to parallel light (hereinafter also referred to as “finitesystem state”) enables further reduction of spherical aberration at thetime of condensing the third wavelength optical beam on the signalrecording face of the third optical disc via the diffraction opticalelement 35 and the object lens 34. While an arrangement has beendescribed here wherein the optical beam of the third wavelength is inputto the diffraction optical element 35 in a state of a predetermineddivergent angle, due to the positional relation between the third lightsource 33 having the third emitting unit for emitting the thirdwavelength optical beam and the collimator lens 42, in the event ofpositioning multiple emitting units at a common light source forexample, this may be realized by providing an element which convertsonly the divergent angle of the optical beam of the third wavelength, orby inputting into the diffraction optical element 35 in a predetermineddivergent angle state by providing a mechanism to drive the collimatorlens 42. Also, the optical beams of the second wavelength, or theoptical beams of the second and third wavelengths, may be input to thediffraction optical element 35 in the finite system state, therebyfurther reducing aberration.

The multi-lens 46 is, for example, a wavelength-selective multi-lens,whereby the returning first through third wavelength optical beamsseparated from the outgoing path optical beams by being reflected at thethird beam splitter 38, after having been reflected off of the signalrecording face of the respective optical disc, and passed through theobject lens 34, diffraction optical element 35, redirecting mirror 44,quarter-wave plate 43, and collimator lens 42, is appropriatelycondensed on the photoreception face of the photodetector or the like ofthe photosensor 45. At this time, the multi-lens 46 provides the returnoptical beam with astigmatism for detection of focus error signals orthe like.

The photosensor 45 receives the return optical beam condensed at themulti-lens 46, and detects, along with information signals, varioustypes of detection signals such as focus error signals, tracking errorsignals, and so forth.

With the optical pickup 3 configured as described above, the object lens34 is driven so as to be displaced based on the focus error signals andtracking error signals obtained by the photosensor 45, whereby theobject lens 34 is moved to a focal position as to the signal recordingface of the optical disc 2, the optical beam is focused onto the signalrecording face of the optical disc 2, and information is recorded to orplayed from the optical disc 2.

The optical pickup 3 is provided on one face of the diffraction opticalelement 35, can provide optical beams of each wavelength with adiffraction efficiency and diffraction angle suitable for each regiondue to the diffraction unit 50 having the first through thirddiffraction regions 51, 52, and 53, can sufficiently reduce sphericalaberration at the signal recording face of the three types of firstthrough third optical discs 11, 12, and 13, of which the format for thethickness of the protective layer differs, and enables reading andwriting of signals to and from the multiple types of optical discs 11,12, and 13, using optical beams of three different wavelengths.

Also, the diffraction optical element 35 having the diffraction unit 50,and object lens 34, in the above optical pickup 3, can function as acondensing optical device for condensing incident optical beams at apredetermined position. In the event of using an optical pickup whichperforms recording and/or playing of information signals by irradiatingoptical beams onto three different types of optical discs, thediffraction unit 50 provided on one face of the diffraction opticalelement 35 enables the condensing optical device to appropriatelycondense corresponding optical beams onto the signal recording face ofthe three types of optical discs in a state with spherical aberrationsufficiently reduced, meaning that three-wavelength compatibility of theoptical pickup using the object lens 34 common to the three wavelengthscan be realized.

Also, while description has been made above regarding a configurationwherein the diffraction optical element 35 to which the diffraction unit50 is provided, and the object lens 34, are provided to an actuator suchas an object lens driving mechanism or the like for driving the objectlens 34 is as to be integral, this may be configured as a condensingoptical unit wherein the diffraction optical element 35 and the objectlens 34 are formed as an integrated unit, in order to improve precisionof assembly to the lens holder of the actuator, and facilitate assemblywork. For example, a condensing optical unit can be configured by use ofspacers or the like to fix the diffraction optical element 35 and objectlens 34 to the holder while setting the positioning, spacing, andoptical axis, so as to be integrally formed. Due to being integrallyassembled to the object lens driving mechanism as described above, thediffraction optical element 35 and object lens 34 can appropriatelycondense the first through third wavelength optical beams on the signalrecording face of the respective optical discs in a state with sphericalaberration reduced, even at the time of field shift such as displacementin the tracking direction.

Next, the optical paths of the optical beams emitted from the firstthrough third light sources 31, 32, and 33 of the optical pickup 3configured as described above, will be described with reference to FIG.2. First, the optical path at the time of emitting the optical beam ofthe first wavelength as to the first optical disc 11 and performingreading or writing of information will be described.

The disc type determination unit 22 which has determined that the typeof the optical disc 2 is the first optical disc 11 causes the opticalbeam of the first wavelength to be emitted from the first emitting unitof the first light source 31.

The optical beam of the first wavelength emitted form the first emittingunit is split into three beams by the first grating 39, for detection oftracking error signals and so forth, and is input to the second beamsplitter 37. The optical beam of the first wavelength which has beeninput to the second beam splitter 37 is reflected at a mirror face 37 athereof, and is output to the third beam splitter 38 side.

The optical beam of the first wavelength which is input to the thirdbeam splitter 38 is transmitted through a mirror face 38 a thereof,output to the collimator lens 42 side, where the divergent angle isconverted by the collimator lens 42 so as to be generally parallellight, provided with a predetermined phase difference at thequarter-wave plate 43, reflected off of the redirecting mirror 44, andoutput to the diffraction optical element 35 side.

The optical beam of the first wavelength which is input to thediffraction optical element 35 is output with the optical beam which haspassed through each region thereof having a predetermined diffractionorder dominant therein as described above, due to the first throughthird diffraction regions 51, 52, and 53 of the diffraction unit 50provided on the incident side face thereof, and input to the object lens34. The optical beam of the first wavelength output from the diffractionoptical element 35 is not only in a state of a predetermined divergentangle, but also is in a state of aperture restriction.

The optical beam of the first wavelength input to the object lens 34 hasbeen input in a divergent angle state whereby spherical aberration ofthe optical beam having passed through the regions 51, 52, and 53 can bereduced, and accordingly is appropriately condensed by the object lens34 on the signal recording face of the first optical disc 11.

The optical beam condensed at the first optical disc 11 is reflected atthe signal recording face, passes through the object lens 34,diffraction optical element 35, redirecting mirror 44, quarter-waveplate 43, and collimator lens 42, is reflected off of the mirror face 38a of the third beam splitter 38, and is output to the photosensor 45side.

The optical beam split from the optical path of the outgoing opticalbeam reflected off of the third beam splitter 38 is condensed on thephotoreception face of the photosensor 45 by the multi-lens 46, anddetected.

Next, description will be made regarding the optical path at the time ofemitting an optical beam of the second wavelength to the second opticaldisc 12 and reading or writing information. The disc type determinationunit 22 which has determined that the type of the optical disc 2 is thesecond optical disc 12 causes the optical beam of the second wavelengthto be emitted from the second emitting unit of the second light source32.

The optical beam of the second wavelength emitted from the secondemitting unit is split into three beams by the second grating 40, fordetection of tracking error signals and so forth, and is input to thefirst beam splitter 36. The optical beam of the second wavelength whichhas been input to the first beam splitter 36 is transmitted through amirror face 36 a thereof, also transmitted through the mirror face 37 aof the second beam splitter 37, and is output to the third beam splitter38 side.

The optical beam of the second wavelength which is input to the thirdbeam splitter 38 is transmitted through the mirror face 38 a thereof,output to the collimator lens 42 side, where the divergent angle isconverted by the collimator lens 42 so as to be generally parallellight, provided with a predetermined phase difference at thequarter-wave plate 43, reflected off of the redirecting mirror 44, andoutput to the diffraction optical element 35 side.

The optical beam of the second wavelength which is input to thediffraction optical element 35 is output with the optical beam which haspassed through each region thereof having a predetermined diffractionorder dominant therein as described above, due to the first throughthird diffraction regions 51, 52, and 53 of the diffraction unit 50provided on the incident side face thereof, and input to the object lens34. The optical beam of the second wavelength output from thediffraction optical element 35 is not only in a state of a predetermineddivergent angle, but also is in a state of aperture restriction due toentering the object lens 34.

The optical beam of the second wavelength input to the object lens 34has been input in a divergent angle state whereby spherical aberrationof the optical beams having passed through the first and seconddiffraction regions 51 and 52 can be reduced, and accordingly isappropriately condensed by the object lens 34 on the signal recordingface of the second optical disc 12.

The return optical path of the optical beam reflected off of the signalrecording face of the second optical disc 12 is the same as with thecase of the above-described optical beam of the first wavelength, andaccordingly description thereof will be omitted.

Next, description will be made regarding the optical path at the time ofemitting an optical beam of the third wavelength to the third opticaldisc 13 and reading or writing information. The disc type determinationunit 22 which has determined that the type of the optical disc 2 is thethird optical disc 13 causes the optical beam of the third wavelength tobe emitted from the third emitting unit of the third light source 33.

The optical beam of the third wavelength emitted form the third emittingunit is split into three beams by the third grating 41, for detection oftracking error signals and so forth, and is input to the first beamsplitter 36. The optical beam of the third wavelength which has beeninput to the first beam splitter 36 is reflected off of the mirror face36 a thereof, transmitted through the mirror face 37 a of the secondbeam splitter 37, and is output to the third beam splitter 38 side.

The optical beam of the third wavelength which is input to the thirdbeam splitter 38 is transmitted through the mirror face 38 a thereof,output to the collimator lens 42 side, where the divergent angle isconverted by the collimator lens 42 so as to be diffused or converged asto generally parallel light, provided with a predetermined phasedifference at the quarter-wave plate 43, reflected off of theredirecting mirror 44, and output to the diffraction optical element 35side.

The optical beam of the third wavelength which is input to thediffraction optical element 35 is output with the optical beam which haspassed through each region thereof having a predetermined diffractionorder dominant therein as described above, due to the first throughthird diffraction regions 51, 52, and 53 of the diffraction unit 50provided on the incident side face thereof, and input to the object lens34. The optical beam of the third wavelength output from the diffractionoptical element 35 is not only in a state of a predetermined divergentangle, but also is in a state of aperture restriction due to having beeninput to the object lens 34.

The optical beam of the third wavelength input to the object lens 34 hasbeen input in a divergent angle state whereby spherical aberration ofthe optical beam having passed through the first diffraction region 51can be reduced, and accordingly is appropriately condensed by the objectlens 34 on the signal recording face of the third optical disc 13.

The return optical path of the optical beam reflected off of the signalrecording face of the third optical disc 13 is the same as with the caseof the above-described optical beam of the first wavelength, andaccordingly description thereof will be omitted.

Note that while a configuration has been described here wherein theoptical beam of the third wavelength has the position of the thirdemitting unit adjusted such that the optical beam of which the divergentangle is converted by the collimator lens 42 and input to thediffraction optical element 35 is in a diffused or converged state as togenerally parallel light, but a configuration may be made wherein theoptical beam is input to the diffraction optical element 35 by providingan element which has wavelength selectivity and converts the divergentangle, or by providing a mechanism which drives the collimator lens 42in the optical axis direction.

Also, while description has been made regarding a configuration whereinthe optical beams of the first and second wavelengths are input to thediffraction optical element 35 in a state of generally parallel light,while the optical beam of the third wavelength is input to thediffraction optical element 35 in a diffused or converged state, thepresent invention is not restricted to this arrangement, andconfigurations may be made wherein, for example, all of the firstthrough third wavelength optical beams are input to the diffractionoptical element 35 in a state of generally parallel light, or whereinany or all of the first through third wavelength optical beams are inputto the diffraction optical element 35 in a diffused or converged state.

The optical pickup 3 to which the present invention has been applied hasfirst through third emitting units for emitting optical beams of firstthrough third wavelengths, an object lens 34 for condensing the opticalbeams of first through third wavelengths emitted from the first throughthird emitting units into a signal recording face of an optical disc,and a diffraction unit 50 provided on one face of an optical elementdisposed on the outgoing optical path of the optical beams of firstthrough third wavelengths, wherein the diffraction unit 50 has firstthrough third diffraction regions 51, 52, and 53, with the first throughthird diffraction regions 51, 52, and 53 being different diffractionstructures ring-shaped and having a predetermined depth, and the firstthrough third diffraction structures whereby optical beams of eachwavelength are diffracted such that diffracted light of a predetermineddiffraction order is dominant as described above, and according to thisconfiguration, optical beams corresponding to each of three types ofoptical discs having difference usage wavelengths can be appropriatelycondensed on the signal recording face using the shared object lens 34,thereby realizing excellent recording and/or playing of informationsignals to/from the respective optical discs by realizingthree-wavelength compatibility with the common object lens 34, withoutnecessitating a complex structure.

That is to say, the optical pickup 3 to which the present invention hasbeen applied obtains optimal diffraction efficiencies and diffractionangels for the first through third wavelength optical beams due to thediffraction unit 50 provided on one face within the optical paththereof, whereby signals can be read from and written to the multipletypes of optical discs 11, 12, and 13, using the optical beams ofdifferent wavelengths emitted from the multiple emitting units providedto each of the light sources 31, 32, and 33, and also optical parts suchas the object lens 34 and so forth can be shared, thereby reducing thenumber of parts, simplifying and reducing the size of the configuration,and realizing high production and lower costs.

Also, the optical pickup 3 to which the present invention has beenapplied can share the object lens 34 between the three wavelengths,thereby preventing trouble of reduction of sensitivity of the actuatordue to increase weight of moving parts. Also, the optical pickup 3 towhich the present invention has been applied can sufficiently reducespherical aberration which is problematic in the case of sharing thecommon object lens 34 between the three wavelengths, due to thediffraction unit 50 provided on one face of the optical element, soproblems such as positioning of diffraction units in the event thatmultiple diffraction units are provided on multiple faces to reducespherical aberration as with the related art, and deterioration ofdiffraction efficiency due to providing of the multiple diffractionunits, can be prevented, which realizes simplification of the assemblyprocess and improved usage efficiency of light.

Further, the optical pickup 3 to which the present invention has beenapplied not only realizes three-wavelength compatibility with thediffraction unit 50 provided on the one face of the diffraction opticalelement 35 described above, but also can perform aperture restrictionwith a numerical aperture corresponding to each of the three types ofoptical discs and three types of optical disc wavelengths, which enablesfurther simplification of configuration, reduction in size, andreduction in costs.

Also, while the above optical pickup 3 has been described having thefirst emitting unit provided at the first light source 31, the secondemitting unit provided at the second light source 32, and the thirdemitting unit provided at the third light source 33, the presentinvention is not restricted to this arrangement, and an arrangement maybe made wherein a light source having two of the first through thirdemitting units, and another light source having the remaining oneemitting unit, are provided at different positions.

Next, description will be made regarding an optical pickup 60 shown inFIG. 19 including a light source having a first emitting unit, and alight source having second and third emitting units. Note that portionsin the following description which are the same as with the opticalpickup 3 will be denoted with the same reference numerals, anddescription thereof will be omitted.

As shown in FIG. 19, the optical pickup 60 to which the presentinvention has been applied includes a first light source 61 having afirst emitting unit for emitting an optical beam of a first wavelength,a second light source 62 having a second emitting unit for emitting anoptical beam of a second wavelength and a third emitting unit foremitting an optical beam of a third wavelength, an object lens 34 forcondensing optical beams emitted from the first through third emittingunits onto the signal recording face of an optical disc 2, and adiffraction optical element 35 provided on the optical path between thefirst through third emitting units and the object lens 34.

Also, the optical pickup 60 includes a beam splitter 63 serving as anoptical path synthesizing unit for synthesizing the optical paths of theoptical beam of the first wavelength that has been emitted from thefirst emitting unit of the first light source 61 and the optical beamsof the second and third wavelengths that have been emitted from thesecond and third emitting unit of the second light source 62, and a beamsplitter 64 serving the same function as the above third beam splitter38.

Further, the optical pickup 60 has a first grating 39, and a grating 65with wavelength dependency, provided between the second light source 62and the beam splitter 63, for diffracting the optical beams of thesecond and third wavelengths that have been emitted from the second andthird emitting units into three beams, for detection of tracking errorsignals and so forth.

Also, the optical pickup 60 has a collimator lens 42, quarter-wave plate43, redirecting mirror 44, photosensor 45, and multi-lens 46, and also acollimator lens driving unit 66 for driving the collimator lens 42 inthe optical axis direction. The collimator lens driving unit 66 canadjust the divergent angle of optical beams passing through thecollimator lens 42 as described above, whereby not only can sphericalaberration be reduced, but in the event that the mounted optical disc isa so-called multi-layer optical disc having multiple signal recordingfaces, recording and/or playing to/from each of the signal recordingfaces is enabled by driving the collimator lens 42 in the optical axisdirection.

With the optical pickup 60 configured as described above, the functionsof each of the optical parts is the same as with the optical pickup 3except for those mentioned above, and the optical paths of the opticalbeams of the first through third wavelengths emitted from the firstthrough third emitting units are the same as with the optical pickup 3except for the above-mentioned, i.e., following synthesizing of theoptical paths of the optical beams of each wavelength by the beamsplitter 64, so detailed description thereof will be omitted.

The optical pickup 60 to which the present invention has been appliedhas first through third emitting units for emitting optical beams offirst through third wavelengths, an object lens 34 for condensing theoptical beams of first through third wavelengths emitted from the firstthrough third emitting units into a signal recording face of an opticaldisc, and a diffraction unit 50 provided on one face of an opticalelement disposed on the outgoing optical path of the optical beams offirst through third wavelengths, wherein the diffraction unit 50 hasfirst through third diffraction regions 51, 52, and 53, with the firstthrough third diffraction regions 51, 52, and 53 being differentdiffraction structures ring-shaped and having a predetermined depth, andthe first through third diffraction structures whereby optical beams ofeach wavelength are diffracted such that diffracted light of apredetermined diffraction order is dominant as described above, andaccording to this configuration, optical beams corresponding to each ofthree types of optical discs having difference usage wavelengths can beappropriately condensed on the signal recording face using the sharedobject lens 34, thereby realizing excellent recording and/or playing ofinformation signals to/from the respective optical discs by realizingthree-wavelength compatibility with the common object lens 34, withoutnecessitating a complex structure. The optical pickup 60 also has theother advantages of the above-described optical pickup 3, as well.

Further, the optical pickup 60 is configured such that the second andthird emitting units are positioned at a common light source 62, therebyrealizing further simplification of structure and reduction in size.Note that in the same way, with the optical pickup to which the presentinvention has been applied, the first through third emitting units maybe positioned at a common light source at generally the same position,thereby realizing further simplification of structure and reduction insize with such a configuration.

The optical disc device 1 to which the present invention has beenapplied has a driving unit for holding and rotationally driving anoptical disc arbitrarily selected from the first through third opticaldiscs, and an optical pickup for performing recording and/or playing ofinformation signals from/to the optical disc being rotationally drivenby the driving unit by selectively irradiating one of multiple opticalbeams of different wavelengths corresponding to the optical disc, and byusing the above-described optical pickups 3 or 60 as the optical pickup,optical beams corresponding to each of three types of optical discshaving difference usage wavelengths can be appropriately condensed onthe signal recording face due to the diffraction unit provided on oneface of the optical element on the optical path of the optical beams ofthe first through third wavelengths, using a common object lens 34,thereby realizing excellent recording and/or playing by realizingthree-wavelength compatibility with the common object lens 34, whileenabling simplification of the structure and reduction in size, withoutnecessitating a complex structure.

<3 > Second Embodiment of Optical Pickup (FIGS. 20 through 36)

Next, an optical pickup 103 to which the present invention is appliedwill be described as a second embodiment of the optical pickup used inthe above-described optical disc device 1, with reference to FIGS. 20through 36. As described above, the optical pickup 103 is an opticalpickup which selectively irradiates multiple optical beams withdifferent wavelengths onto optical discs arbitrarily selected from firstthrough third optical discs 11, 12, and 13, of which the format such asthe thickness of the protective layer differs, thereby performingrecording and/or playing of information signals.

As shown in FIG. 20, the optical pickup to which the present inventionhas been applied includes a first light source 131 having a firstemitting unit for emitting an optical beam of a first wavelength, asecond light source 132 having a second emitting unit for emitting anoptical beam of a second wavelength longer than the first wavelength, athird light source 133 having a third emitting unit for emitting anoptical beam of a third wavelength longer than the second wavelength, anobject lens 134 for condensing optical beams emitted from the emittingunit of the first through third emitting units onto the signal recordingface of an optical disc 2, and a diffraction optical element 135provided on the optical path between the first through third emittingunits and the object lens 134.

Also, the optical pickup 103 includes a first beam splitter 136 providedbetween the second and third emitting units and the diffraction opticalelement 135, serving as an optical path synthesizing unit forsynthesizing the optical paths of the optical beam of the secondwavelength that has been emitted from the second emitting unit and theoptical beam of the third wavelength that has been emitted from thethird emitting unit, a second beam splitter 137 provided between thefirst beam splitter 136 and the diffraction optical element 135, servingas an optical path synthesizing unit for synthesizing the optical pathof the optical beams of the second and third wavelengths of which theoptical paths have been synthesized by the first beam splitter 136 andthe optical beam of the first wavelength emitted form the frits emittingunit, and a third beam splitter 138 provided between the second beamsplitter 137 and the diffraction optical element 135, serving as anoptical path splitting unit for splitting the outgoing optical path ofthe optical beams of the first through third wavelengths synthesized atthe second beam splitter 137 from the returning optical path of theoptical beams of the first through third wavelengths reflected off ofthe optical disc (hereinafter also referred to as “return path”).

Further, the optical pickup 103 has a first grating 139 provided betweenthe first emitting unit of the first light source unit 131 and thesecond beam splitter 137, for diffracting the optical beam of the firstwavelength that has been emitted from the first emitting unit into threebeams, for detection of tracking error signals and so forth, a secondgrating 140 provided between the second emitting unit of the secondlight source unit 132 and the first beam splitter 136, for diffractingthe optical beam of the second wavelength that has been emitted from thesecond emitting unit into three beams, for detection of tracking errorsignals and so forth, and a third grating 141 provided between the thirdemitting unit of the third light source unit 133 and the first beamsplitter 136, for diffracting the optical beam of the third wavelengththat has been emitted from the third emitting unit into three beams, fordetection of tracking error signals and so forth.

Also, the optical pickup 103 has a collimator lens 142 provided betweenthe third beam splitter 138 and the diffraction optical element 135,serving as a divergent angle conversion unit for converting thedivergent angle of the optical beams of the first through thirdwavelengths of which the optical paths have been synthesized at thethird beam splitter 138 so as to be adjusted into a state of generallyparallel light or a state diffused or converged as to generally parallellight, and outputting, a quarter-wave plate 143 provided between thecollimator lens 142 and the diffraction optical element 135, so as toprovide quarter-wave phase difference to the optical beams of the firstthrough third wavelengths of which the divergent angle has been adjustedby the collimator lens 142, and a redirecting mirror 144 providedbetween the diffraction optical element 135 and the quarter-wave plate143, for redirecting by reflecting the optical beam which has passedthrough the above-described optical parts within a plane generallyorthogonal to the optical axis of the object lens 134 and diffractionoptical element 135, so as to emit the optical beam in the optical axisdirection toward the object lens 134 and diffraction optical element135.

Further, the optical pickup 103 includes a photosensor 145 for receivingand detecting the optical beams of the first through third wavelengthssplit at the third beam splitter 138 on the return path from the opticalbeams of the first through third wavelengths on the outgoing path, and amulti lens 146 provided between the third beam splitter 138 and thephotosensor 145, for condensing optical beams of the first through thirdwavelengths split at the third beam splitter 138 return path onto thephotoreception face of a photodetector or the like of the photosensor145, and also providing astigmatism for detecting focus error signals orthe like.

The first light source 131 has a first emitting unit for emitting anoptical beam of a first wavelength around 405 nm onto the first opticaldisc 11. The second light source 132 has a second emitting unit foremitting an optical beam of a second wavelength around 655 nm onto thesecond optical disc 12. The third light source 133 has a third emittingunit for emitting an optical beam of a third wavelength around 785 nmonto the third optical disc 13. Note that while the first through thirdemitting units are configured disposed at individual light sources 131,132, and 133, the invention is not restricted to this, and anarrangement may be made wherein two emitting units of the first throughthird emitting units are disposed at one light source and the remainingemitting unit is disposed at another light source at a differentposition, or wherein the first through third emitting units are disposedso as to form a light source at generally the same position.

The object lens 134 condenses the input optical beams of the firstthrough third wavelengths into the signal recording face of the opticaldisc 2. The object lens 134 is movably held by an object lens drivingmechanism such as an unshown biaxial actuator or the like. The objectlens 134 is driven along two axes, one in the direction toward/away fromthe optical disc 2, and the other in the radial direction of the opticaldisc 2, by being moved by a biaxial actuator or the like based on thetracking error signals and focus error signals generated from the RFsignals of the return light from the optical disc 2 that has beendetected at the photosensor 145. The object lens 134 condenses opticalbeams emitted from the first through third emitting units such that theoptical beams are always focused on the signal recording face of opticaldisc 2, and also causes the condensed optical beam to track a recordingtrack formed on the signal recording face of the optical disc 2. Notethat a configuration wherein the later-described diffraction opticalelement 135 is held by a lens holder of the object lens drivingmechanism where the object lens 134 is held so as to be integral withthe object lens 134 enables the later-described advantages of adiffraction unit 150 provided to the diffraction optical element 135 tobe suitably manifested at the time of field shift of the object lens 134such as movement in the tracking direction.

The diffraction optical element 135 has, as one face thereof forexample, a diffraction unit 150 having multiple diffraction regions onthe incident side face thereof, with the diffraction unit 150diffracting each of the optical beams of the first through thirdwavelengths passing through each of the multiple diffraction regionsinto predetermined orders and inputting into the object lens 134, i.e.,inputting into the object lens 134 as optical beams in a diffused stateor converged state having a predetermined divergent angle, whereby thesingle object lens 134 can be used to perform suitable condensing of theoptical beams of the first through third wavelengths such that sphericalaberration does not occur at the signal recording face of the threetypes of optical discs corresponding to the optical beams of the firstthrough third wavelengths. The diffraction optical element 135 serves asa condensation optical device along with the object lens 134 toappropriately perform condensation such that no spherical aberrationoccurs at the signal recording face of the three types of optical discscorresponding to the optical beams of the three different wavelengths.

The diffraction optical element 135 having the diffraction unit 150performs diffraction of the first wavelength optical beam BB0 which hastransmitted the diffraction unit 150 so as to become +1st orderdiffracted beam BB1 and inputs to the object lens 134, i.e., as anoptical beam in a diffused state having a predetermined divergent angle,thereby appropriately condensing on the signal recording face of thefirst optical disc 11, as shown in FIG. 21A, performs diffraction of thesecond wavelength optical beam BD0 which has transmitted the diffractionunit 150 so as to become −1st order diffracted beam BD1 and inputs tothe object lens 134, i.e., as an optical beam in a converged statehaving a predetermined divergent angle, thereby appropriately condensingon the signal recording face of the second optical disc 12, as shown inFIG. 21B, and performs diffraction of the third wavelength optical beamBC0 which has transmitted the diffraction unit 150 so as to become −2ndorder diffracted beam BC1 and inputs to the object lens 134, i.e., as anoptical beam in a converged state having a predetermined divergentangle, thereby appropriately condensing on the signal recording face ofthe third optical disc 13, as shown in FIG. 21C, for example, wherebysuitable condensation can be performed such that no spherical aberrationoccurs at the signal recording face of the three types of optical discs,with a single object lens 134. While description has been made here withan example wherein optical beams of the same wavelength are made to bediffracted beams of the same diffraction order at the multiplediffraction regions of the diffraction unit 150, with reference to FIGS.21A through 21C, the diffraction unit 150 configuring the optical pickup103 to which the present invention is applied enables diffraction ordercorresponding to each wavelength to be set for each region as describedlater, so as to further reduce spherical aberration.

Note that in the above and following description of diffraction orders,an order of diffraction which draws closer to the optical axis side inthe direction of progression with regard to an input optical beam is apositive order. In other words, an order which diffracts toward theoptical axis direction of the input optical beam is a positive order.That is to say, with the above first through third wavelengths, +1 orderdiffracted light selected so as to be dominant diffracts in thedirection of convergence as compared to the input optical beams of eachwavelength.

Specifically, as shown in FIG. 22, the diffraction unit 150 provided atthe incident side face of the diffraction optical element 135 has agenerally-circular first diffraction region 151 provided on theinnermost portion (hereinafter also referred to as “inner ring zone”), aring-shaped second diffraction region 152 provided on the outer side ofthe first diffraction region 151 (hereinafter also referred to as“middle ring zone”), and a ring-shaped third diffraction region 153provided on the outer side of the second diffraction region 152(hereinafter also referred to as “outer ring zone”).

The first diffraction region 151 which is an inner ring zone has a firstdiffraction structure formed having a ring shape with a predetermineddepth, and diffracts the optical beam of the first wavelength that istransmitted therethrough such that diffracted light of an order whichforms an appropriate spot condensed on the signal recording face of thefirst optical disc via the object lens 134 is dominant, i.e., such thatmaximum diffraction efficiency is manifested regarding diffracted lightof other orders.

The first diffraction region 151 diffracts the optical beam of thesecond wavelength that is transmitted therethrough such that diffractedlight of an order which forms an appropriate spot condensed on thesignal recording face of the second optical disc via the object lens 134is dominant, i.e., such that maximum diffraction efficiency ismanifested regarding diffracted light of other orders, by way of thefirst diffraction structure.

The first diffraction region 151 diffracts the optical beam of the thirdwavelength that is transmitted therethrough such that diffracted lightof an order which forms an appropriate spot condensed on the signalrecording face of the third optical disc via the object lens 134 isdominant, i.e., such that maximum diffraction efficiency is manifestedregarding diffracted light of other orders, by way of the firstdiffraction structure.

Thus, the first diffraction region 151 has a diffraction structuresuitably formed whereby diffracted light of a predetermined order isdominant in the optical beam of each wavelength, thereby enablingcorrection and reduction of spherical aberration at the time of opticalbeams of each wavelength that have passed through the first diffractionregion 151 and become diffracted light of a predetermined order beingcondensed on the signal recording face of the respective optical discsby the object lens 134.

Specifically, as shown in FIGS. 22 and 23A, the first diffraction region151 is formed with the cross-sectional form of ring shapes centered onthe optical axis being formed in a blazed shape having a predetermineddepth (hereinafter also referred to as “groove depth”) d. Note that thecross-sectional form of the ring shapes in this diffraction structuremeans the cross-sectional form of the rings taken along a planeincluding the radial direction of the rings, i.e., a plane orthogonal tothe tangential direction of the rings. Also, in FIG. 23A, the saw-toothshape is formed such that the slopes of the protrusions and recessesheat inward in the radial direction, which is to make the selecteddiffraction order positive, and obtain a converged state with a desireddivergent angle. Note that here, a divergent angle for obtaining aconverged state is a negative divergent angle. The symbol R_(o) in FIGS.23A through 23C represents the direction toward the outer side in theradial direction of the rings, i.e., the direction away from the opticalaxis.

Note that in the first diffraction structure formed at the firstdiffraction region 151, the groove width is determined taking intoconsideration the dominant diffraction order and diffraction efficiency.Also, as shown in FIG. 23A, the groove width is smaller in value thefarther away from the optical axis. Note that the groove widths aredetermined based on phase difference obtained at the diffraction regionsformed with the groove widths, such that the spot condensed on thesignal recording face of the optical disc is optimal.

Also, in a case wherein the first diffraction region 151 diffracts theoptical beam of the first wavelength which is transmitted therethroughsuch that diffracted light of the k1 i'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, diffracts theoptical beam of the second wavelength which is transmitted therethroughsuch that diffracted light of the k2 i'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, and diffracts theoptical beam of the third wavelength which is transmitted therethroughsuch that diffracted light of the k3 i'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, k1 i, k2 i, and k3iare such that (k1 i, k2 i, k3 i)=(+1, +1, +1).

Now, as a first aspect regarding the first diffraction region 151, thereis need to reduce spherical aberration at each wavelength, as a secondaspect, there is the need to take into considerationtemperature-spherical aberration properties, i.e., there is the need toreduce spherical aberration occurring during temperature change, and asa third perspective, the structure must be advantageous inmanufacturing, and from these, the above diffraction orders k1 i, k2 i,and k3 ihave been selected as diffraction orders with maximumdiffraction efficiency, a point which will be described below.

First, the first perspective will be described. Generally, in a regionhaving a function such as the first diffraction region 151, it is knownthat satisfying the conditional expression(λ1×k1x−λ2×k2x)/(t1−t2)≈(λ1×k1x−λ3×k3x)/(t1−t3)

where

-   -   λ1 is the first wavelength (nm),    -   λ2 is the second wavelength (nm),    -   λ3 is the third wavelength (nm),    -   k1 iis the diffraction order where an optical beam of the first        wavelength is selected,    -   k2 iis the diffraction order where an optical beam of the second        wavelength is selected,    -   k3 iis the diffraction order where an optical beam of the third        wavelength is selected,    -   t1 is the thickness (mm) of the first protective layer of the        first optical disc,    -   t2 is the thickness (mm) of the second protective layer of the        second optical disc,    -   t3 is the thickness (mm) of the third protective layer of the        third optical disc, and    -   x=i for the inner ring zone in k1 x, k2 x, and k3 x in this        conditional expression,

is a condition whereby spherical aberration on the signal recording faceof each optical disc at each wavelength can be corrected and reduced. Inthe first diffraction region 151 which is the above-described inner ringzone, when λ1=405 (nm), λ2=655 (nm), λ3=785 (nm), t1=0.1 (mm), t2=0.6(mm), and t3=1.1 (mm), then k1 i=+1, k2 i=+1, and k3 i=+1, each hold,thereby satisfying the conditional expression, and it has been confirmedthat spherical aberration can be reduced. This can be restated in otherwords that when plotting points Pλ1, Pλ2, and Pλ3 in the graph in FIG.24 wherein the horizontal axis represents a value calculated bywavelength×diffraction order (nm) and the vertical axis represents thethickness (mm) of the protective layer, in the event of being situatedon a straight line this means that the spherical aberration on thesignal recording face of each optical disc of each wavelength can becorrected and reduced; in reality, in the case of plotting the pointsPλ1, Pλ2, and Pλ3 under the conditions described below, the points areon a generally straight design line, meaning that spherical aberrationcan be realized. Specifically, the object lens 134 has the material ofwhich it is configured, and the face shape at the input and outputsides, determined with the line L11 in FIG. 24 as the design line, withthe inclination of the design line approximating the inclination of theline connecting Pλ1 and Pλ2 calculated by (t1−t2)/(λ1×k1 x−λ2×k2 x) orthe inclination of the line connecting Pλ1 and Pλ3 calculated by(t1−t3)/(λ1×k1 x−λ3×k3 x), or determined taking into consideration theinclination of these lines and other design conditions. Note that whilein FIG. 24 Pλ3 deviates slightly upwards from the line, sphericalaberration can be corrected in a sure manner by inputting the incidentlight to the one of the object lens 134 and diffraction optical element135 which is closer to the emitting units, which is the diffractionoptical element 135 in this case, as a divergent ray.

Next, the second perspective will be described. In a region having afunction such as the first diffraction region 151, these orders must bepositive in order to realize suitable temperature-spherical aberrationproperties, i.e., reduction in spherical aberration without depending ontemperature change. Now, a positive diffraction order is an order ofdiffraction which draws closer to the optical axis side in the directionof progression with regard to an input optical beam. Sphericalaberration which occurs due to rise in temperature is represented as thesum of an effect term ΔWn due to refractive index fluctuation of thematerial configuring the object lens 134 under change in temperature,and an effect term ΔWλ due to wavelength fluctuation of the incidentoptical beam under change in temperature, i.e., by ΔW which is obtainedby the relational expressionΔW=ΔWn+ΔWλ.

Of these, the sign of the latter effect term ΔWλ due to wavelengthfluctuation is governed by the diffraction direction due to thediffraction unit 150. The object lens 134 provides positive power(refractive power), so the refractive index drops as the temperaturerises, consequently acting in the direction such that the positive poweris weakened, and the effect term Wn due to refractive index fluctuationis ΔWn<0. There is the need for the effect term ΔWλ due to wavelengthfluctuation to be such that ΔWλ>0 holds in order to cancel out thiseffect term Wn, i.e., such that the positive power is increased at thediffraction unit 150 under rising temperature. Accordingly, it isadvantageous form the perspective of temperature-spherical aberrationproperties for the diffraction orders at the diffraction unit 150 to bepositive.

Now, the fact that the spherical aberration occurring due to temperaturerise can be cancelled out due to a configuration such as described willbe described in further detail with reference to the longitudinalaberration drawing in FIGS. 25A through 25C. Prior to description withreference to FIGS. 25A through 25C, longitudinal aberration will bedescribed with reference to FIGS. 26A and 26B. In FIGS. 26A and 26B thex-axial direction represents the optical axis direction, and the y-axialdirection represents the image height, i.e., the height from the opticalaxis in the direction orthogonal to the optical axis.

As shown in FIG. 26A, generally, optical beams passing through a lenswith no aberration are condensed on the same image plane regardless ofthe incident position in the direction orthogonal to the optical axis ofthe lens, i.e., condensed equally at the paraxial image point A0.

On the other hand, as shown in FIG. 26B for example, optical beamspassing through a lens with aberration are condensed on different imageplanes according to the incident position in the direction orthogonal tothe optical axis of the lens, i.e., condensed at positions shifted inthe x-axial direction from the paraxial image point B0. At this time,the line LB indicating the state of longitudinal aberration is indicatedby a curve obtained by connecting points B1 through B7 for example, withthe height of the incident position of the optical beam from the opticalaxis (image height) as the y-axis, and the position where the imageplane of rays input at the position this height from the optical axisand the optical axis which is the principal ray intersect as the x-axis.Specifically, the ray input at the height position y1 from the opticalaxis intersects with the optical axis at the position x1, so a B1 at thecoordinates (x1, y1) is obtained. Also, the ray input at the heightposition y2 from the optical axis intersects with the optical axis atthe position x2, so a B2 at the coordinates (x2, y2) is obtained. Thisholds true for B3 through B7 as well, so detailed description will beomitted here.

In the same way, with the lens shown in FIG. 26A, in the same way aswith the above-described line LB, the line LA indicating the state oflongitudinal aberration is indicated by a line obtained by connectingpoints A1 through A7 for example, with the height of the incidentposition of the optical beam from the optical axis as the y-axis, andthe position where rays input at the position this height from theoptical axis, and the optical axis, intersect as the x-axis. In the casein FIG. 26A, the position of the x-axis intersecting with the opticalaxis is always constant regardless of the position on the y-axis, so theline LA indicating the state of longitudinal aberration agrees with they-axis. Generally, a line indicating the state of longitudinalaberration can be said to be representing a state of little or notaberration of in a state of matching the y-axis as shown in FIG. 26A orin a state as close thereto as possible.

Next, in light of the above, the fact that the spherical aberrationoccurring due to rise in temperature can be cancelled out by selectingthe above-described diffraction orders k1 i, k2 i, and k3 iwill bedescribed with reference to FIGS. 25A through 25C.

FIGS. 25A and 25B are conceptual diagrams illustrating the effect termΔWn due to refractive power fluctuation of the composition materialunder change in temperature, and the effect term ΔWλ due to wavelengthfluctuation of the incident optical beam under change in temperature, aslongitudinal aberration respectively. In FIGS. 25A and 25B, the dottedline Lwn represents the longitudinal aberration due to refractive powerfluctuation, i.e., represents the effect term ΔWn due to refractivepower fluctuation of the composition material under change intemperature as longitudinal aberration, the single-dot broken line Lwλ1represents longitudinal aberration due to change in diffraction angle inthe event that the selected diffraction order is a positive diffractionorder which is to say positive refractive power is provided by thediffraction unit, i.e., the effect term ΔWλ due to wavelengthfluctuation as longitudinal aberration, and the single-dot broken lineLwλ2 represents longitudinal aberration in the event that the selecteddiffraction order is a negative diffraction order which is to saynegative refractive power is provided by the diffraction unit, i.e., theeffect term ΔWλ due to wavelength fluctuation as longitudinalaberration, for comparison with Lwλ1. In FIGS. 25A and 25B, the solidlines Lw1 and Lw2 represent spherical aberration ΔW occurring due totemperature rise, obtained by adding the ΔWn and ΔWλ in FIG. 25A, aslongitudinal aberration. In FIG. 25B, the solid line Lw1 illustratesaddition of the dotted line Lwn in FIG. 25B and the single-dot brokenline Lwλ1, i.e., the spherical aberration ΔW in the event that thediffraction order is positive, and the solid line Lw2 In FIG. 25Aillustrates addition of the dotted line Lwn in FIG. 25A and thesingle-dot broken line Lwλ2, i.e., the spherical aberration ΔW in theevent that the diffraction order is negative.

As shown in FIG. 25B, in a region having a function such as the firstdiffraction region 151, selecting the above-described diffraction ordersk1 i, k2 i, and k3 i, i.e., selecting positive diffraction ordersenables a situation wherein aberration is suppressed, with thelongitudinal aberration state (Lw1) being close to the state in FIG.26A. Conversely, in the event that negative diffraction orders areselected as shown in FIG. 25A, the state of longitudinal aberration(Lw2) does not have aberration suppressed. That is to say, this is aproblematic state from the perspective of temperature-sphericalaberration properties. As described above, selecting the above-describeddiffraction orders k1 i, k2 i, and k3 iis advantageous from theperspective of temperature-spherical aberration properties.

Next, the third perspective will be described. A diffraction unit havinga function such as the first diffraction region 151 is configured withone face of the diffraction optical element 135, or one face of theobject lens as described later, having a diffraction structure formedthereupon, so in the event that the diffraction order selected is verygreat, the depth d of the diffraction structure to be formed becomesdeep. A deep diffraction structure depth d may not only lead to poorformation precision; the optical path length increasing effect due totemperature change is greater, and there may be a problem wherein thetemperature-diffraction efficiency property deteriorates. Due to suchreasons, diffractions orders up to around the 3rd order or 4th order issuitable, and generally used. That is to say, the diffraction orders k1i, k2 i, and k3 i, to be selected for the first diffraction region 151are such as described above, so from the perspective of manufacturing aswell, manufacturing is easy, there is no problem with deterioration inprecision or the like, quality can be improved, and consequently,diffracted light having excellent diffraction efficiency can be emittedin a sure manner.

Thus, the first diffraction region 151 serving as the inner ring zonehas selected excellent orders from the first perspective of reduction ofspherical aberration, the second perspective of temperature-sphericalaberration properties, and the third perspective from depth of thediffraction structure formed in manufacturing, and accordingly, theabove configuration yields a configuration wherein spherical aberrationcan be reduced, occurrence of aberration under temperature change can bereduced, and which is advantageous in manufacturing.

The second diffraction region 152 which is a middle ring zone has asecond diffraction structure formed which is ring shaped and has apredetermined depth, and which is a different structure from the firstdiffraction structure. The second diffraction region 152 diffracts theoptical beam of the first wavelength that is transmitted therethroughsuch that diffracted light of an order which forms an appropriate spotcondensed on the signal recording face of the first optical disc via theobject lens 134 is dominant, i.e., such that maximum diffractionefficiency is manifested regarding diffracted light of other orders.

The second diffraction region 152 diffracts the optical beam of thesecond wavelength that is transmitted therethrough such that diffractedlight of an order which forms an appropriate spot condensed on thesignal recording face of the second optical disc via the object lens 34is dominant, i.e., such that maximum diffraction efficiency ismanifested regarding diffracted light of other orders, by way of thesecond diffraction structure.

The second diffraction region 152 diffracts the optical beam of thethird wavelength that is transmitted therethrough such that diffractedlight of orders other than an order which forms an appropriate spotcondensed on the signal recording face of the third optical disc via theobject lens 134 is dominant, i.e., such that maximum diffractionefficiency is manifested regarding diffracted light of other orders, byway of the second diffraction structure. Note that the seconddiffraction region 152 can sufficiently reduce diffraction efficiencydiffracted light of an order which forms an appropriate spot condensedon the signal recording face of the third optical disc via the objectlens 134 for the optical beam of the third wavelength that istransmitted therethrough, by way of the second diffraction structure.

Thus, the second diffraction region 152 has a diffraction structureformed suitably whereby diffracted light of a predetermined order isdominant in the optical beam of each wavelength, thereby enablingcorrection and reduction of spherical aberration at the time of opticalbeams of first and second wavelengths that have passed through thesecond diffraction region 152 and become diffracted light of apredetermined order being condensed on the signal recording face of therespective optical discs by the object lens 134.

Also, the second diffraction region 152 is configured so as to functionas described above regarding the optical beams of the first and secondwavelengths, but such that diffracted light of orders other thandiffracted light of an order which is condensed on the signal recordingface of the third optical disc after passing through the seconddiffraction region 152 and the object lens 134 is dominant, wherebyaperture restriction can be applied to the optical beam of the thirdwavelength, such that even if the optical beam of the third wavelengthwhich has been transmitted through the second diffraction region 152 isinput to the object lens 134, there is very little effect on the signalrecording face of the third optical disc, i.e., markedly reducing thelight quantity of the optical beam of the third wavelength which iscondensed on the signal recording face of the third optical disc afterpassing through the second diffraction region 152 and the object lens134, to around zero.

Now, the above-described first diffraction region 151 is formed of asize such that the optical beam of the third wavelength which has beentransmitted through the region thereof is input to the object lens 134in the same state as an optical beam which has been subjected toaperture restriction at around NA=0.45, and since the second diffractionregion 152 formed on the outer side of the first diffraction region 151does not allow condensation of the optical beam of the third wavelengthwhich has been transmitted through this region on the third optical discvia the object lens 134, the diffraction unit 150 which has the firstand second diffraction regions 151 and 152 configured thus functions soas to restrict the numerical aperture of the optical beam of the thirdwavelength to around NA=0.45. It should be noted however, that while inthis arrangement of the diffraction unit 150, the optical beam of thethird wavelength is subjected to aperture restriction around NA=0.45,but numerical aperture restriction due to the above configuration is notlimited to this.

Specifically, as shown in FIGS. 22 and 23A, in the same way as with theabove-described first diffraction region 151, the second diffractionregion 152 is formed with the cross-sectional form of ring shapescentered on the optical axis being formed in a blazed shape having apredetermined depth d.

Also, while description is made here with regard to the seconddiffraction region 152 having the cross-sectional form of the ringsformed as a diffraction structure with a blazed form, any diffractionstructure may be used as long as an optical beam of a predeterminedorder is dominant as to the optical beam of each wavelength as describedabove, so a configuration may be used such as shown in FIG. 23B, with adiffraction region 152B having a diffraction structure wherein thecross-sectional form of the rings is formed with the cross-sectionalform of ring shapes centered on the optical axis being formed in astaircase-like shape having a predetermined depth d and a predeterminednumber of steps S, continuing in the radial direction in a staircaseform, for example.

Now, the diffraction structure having a staircase-like shape with apredetermined number of steps S is a structure wherein a staircase-likeshape having first through S'th steps of which the depths areapproximately the same is configured continuing in the radial direction,and further, in other words, is a structure having first through S+1'thdiffraction faces formed at approximately the same interval in theoptical axis direction. Also, the predetermined depth d in thediffraction structure means the length along the optical axis directionbetween the diffraction face of the S+1'th diffraction face which isformed at the side of the staircase form closest to the surface (i.e.,the highest step, which is the shallowest position) and diffraction faceof the first diffraction face which is formed at the side of thestaircase form closest to the element (i.e., the lowest step, which isthe deepest position). This holds true for later-described FIG. 23C aswell. Note that while a structure has been illustrated in FIGS. 23B and23C wherein the steps of each stepped portion of the staircase shape areformed such that the closer to the inner side in the radial direction,the closer to the surface side the steps are formed, an arrangementwhich has been made to select positive diffraction orders, and to obtaina convergent state with a desired divergent angle. In the second and thelater-described third diffraction structures, the groove depth d andnumber of steps S in the case of having a staircase form are determinedtaking into consideration the dominant diffraction order and diffractionefficiency.

Also, as shown in FIGS. 23B and 23C, the groove width of each step (theradial-direction dimension of each step portion of the staircase form)is such that the steps are formed with equal width within one staircaseform, while looking at the different staircase forms formed continuouslyin the radial direction, the value of the step width is greater atstaircase forms further away form the optical axis. Note that the groovewidths are determined based on phase difference obtained at thediffraction regions formed with the groove widths, such that the spotcondensed on the signal recording face of the optical disc is minimal.

For example, the diffraction structure of the second diffraction region152B is, as shown in FIG. 23B, a diffraction structure having astaircase portion including first through third steps 152 s 1, 152 s 2,and 152 s 3, formed continuously in the radial direction, wherein thenumber of steps is 3 (S=3), and the depth of each step is generally thesame depth (d/3), and first through fourth diffraction faces 152 f 1,152 f 2, 152 f 3, and 152 f 4 formed at the same intervals of d/3 in theoptical axis direction.

Also, in a case wherein the second diffraction region 152 diffracts theoptical beam of the first wavelength which is transmitted therethroughsuch that diffracted light of the k1 m'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, and diffracts theoptical beam of the second wavelength which is transmitted therethroughsuch that diffracted light of the k2 m'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, the diffractionorders k1 mand k2 mare in the following relation.(k1m,k2m)=(+1,+1),(+3,+2).

Now, the second diffraction region 152 serving as the inner ring zone isobtained by orders most excellent from the first through thirdperspectives described in the above description of the first diffractionregion 151, and accordingly, spherical aberration can be reduced,occurrence of aberration under temperature change can be reduced, aconfiguration which is advantageous in manufacturing can be had.

Now, as described above, the second diffraction region 152 is configuredso as to diffract light such that the diffraction efficiency of thediffracted light of the diffraction orders k1 mand k2 mfor the opticalbeams of the first and second wavelengths passing through the objectlens 134 is in a high state, so as to form a suitable spot condensed onthe signal recording faces of the first and second optical discs, andalso to have an aperture restriction function for suppressing thediffraction efficiency of the diffraction order of the optical beam ofthe third wavelength to be condensed on the signal recording face of thethird optical disc as much as possible, but a configuration may be madewherein the optical beam of this diffraction order in the optical beamof the third wavelength is shifted from a state wherein the focal pointis imaged on the signal recording face of the third optical disc, so asto further reduce the light quantity of the optical beam substantiallycondensed on the third optical disc. Note that hereinafter, shifting theposition where the optical beam of a predetermined wavelength is imagedvia the object lens 34, from the signal recording face of thecorresponding optical disc, so as to substantially reduce the lightquantity of the optical beam condensed on the signal recording face,will be also referred to as “flaring”.

Now, with regard to the second diffraction region 152, flaring and theconfiguration thereof will be described. Description has been made aboveregarding the first diffraction region 151 that there is the need tosatisfy the conditional expression of(λ1×k1x−λ2×k2x)/(t1−t2)≈(λ1×k1x−λ3×k3x)/(t1−t2),

this conditional expression (x=m for the middle ring zone in k1 x, k2 x,and k3 x in this conditional expression) being taken into considerationin the second diffraction region 152 as well. With this seconddiffraction region 152 serving as the middle ring zone, giving thoughtto the function of diffracting light such that the diffractionefficiency of the diffracted light of the diffraction orders k1 mand k2mfor the optical beams of the first and second wavelengths passingthrough the object lens 134 is in a high state, so as to form a suitablespot on the signal recording faces of the first and second opticaldiscs, as described above, the Pλ1 and Pλ2 to be plotted can bepositioned on a design line, and further, a design line can be selectedsuch that the Pλ3 intentionally deviates from the design line so as tocause flaring regarding the third wavelength. That is to say,configuring the object lens 134 formed based on a design line whereinPλ3 deviates from the design line allows the diffracted rays of thediffraction order of the optical beam of the third wavelength to beshifted from a state of imaging the focal point on the signal recordingface of the third optical disc, so the quantity of light of the opticalbeam of the third wavelength condensed on the signal recording face ofthe third optical disc can be substantially reduced, and accordingly,aperture restriction regarding the optical beam of the third wavelengthas described above can be performed in a sure an excellent manner.Specifically, in the event that (k1 m, k2 m, k3 m)=(+3, +2, +2) asdescribed later with reference to FIG. 33, Pλ3 deviates from the designline L13, so in addition to the effect of reducing the diffractionefficiency of the order of the third wavelength, due to the diffractionstructure formed on the second diffraction region 152 which is aninitially expected advantage, the advantages of flaring can also beobtained, thereby enabling the quantity of light of the optical beam ofthe third wavelength input to the third optical disc to be furthersuppressed.

The third diffraction region 153 which is an outer ring zone has a thirddiffraction structure formed which is ring shaped and has apredetermined depth, and which is a different structure from the firstand second diffraction structures. The third diffraction region 153diffracts the optical beam of the first wavelength that is transmittedtherethrough such that diffracted light of an order which forms anappropriate spot condensed on the signal recording face of the firstoptical disc via the object lens 134 is dominant, i.e., such thatmaximum diffraction efficiency is manifested regarding diffracted lightof other orders.

Also, the third diffraction region 153 diffracts the optical beam of thesecond wavelength that is transmitted therethrough such that diffractedlight of orders other than an order which forms an appropriate spotcondensed on the signal recording face of the second optical disc viathe object lens 134 is dominant, i.e., such that maximum diffractionefficiency is manifested regarding diffracted light of other orders, byway of the third diffraction structure. Note that the third diffractionregion 153 diffracts the optical beam of the second wavelength that istransmitted therethrough such that diffraction efficiency of diffractedlight of an order which forms an appropriate spot condensed on thesignal recording face of the second optical disc via the object lens 134is sufficiently reduced, by way of the third diffraction structure.

Also, the third diffraction region 153 diffracts the optical beam of thethird wavelength that is transmitted therethrough such that diffractedlight of orders other than an order which forms an appropriate spotcondensed on the signal recording face of the third optical disc via theobject lens 134 is dominant, i.e., such that maximum diffractionefficiency is manifested regarding diffracted light of other orders, byway of the third diffraction structure. Note that the third diffractionregion 153 can sufficiently reduce diffraction efficiency diffractedlight of an order which forms an appropriate spot on the signalrecording face of the third optical disc via the object lens 134 for theoptical beam of the third wavelength that is transmitted therethrough,by way of the third diffraction structure.

Thus, the third diffraction region 153 has a diffraction structuresuitably formed whereby diffracted light of a predetermined order isdominant in the optical beam of each wavelength, thereby enablingcorrection and reduction of spherical aberration at the time of theoptical beam of the first wavelength that has passed through the thirddiffraction region 153 and become diffracted light of a predeterminedorder being condensed on the signal recording face of the optical discby the object lens 134.

Also, the third diffraction region 153 is configured so as to functionas described above regarding the optical beams of the first wavelength,but such regarding optical beams of the second and third wavelength thatdiffracted light of orders other than diffracted light of an order whichis condensed on the signal recording face of the second and thirdoptical discs after passing through the third diffraction region 153 andthe object lens 134 is dominant, whereby aperture restriction can beapplied to the optical beam of the second wavelength, such that even ifthe optical beam of the second and third wavelengths which have beentransmitted through the third diffraction region 153 is input to theobject lens 134, there is very little effect on the signal recordingface of the third optical disc, i.e., markedly reducing the lightquantity of the optical beam of the third wavelength which is condensedon the signal recording face of the second and third optical discs afterpassing through the third diffraction region 153 and the object lens134, to around zero. Note that the third diffraction region 153 canfunction so as to perform aperture restriction for the optical beam ofthe third wavelength, along with the above-described second diffractionregion 152.

Now, the above-described second diffraction region 152 is formed of asize such that the optical beam of the second wavelength which has beentransmitted through the region thereof is input to the object lens 134in the same state as an optical beam which has been subjected toaperture restriction at around NA=0.6, and since the third diffractionregion 153 formed on the outer side of the second diffraction region 152does not allow condensation of the optical beam of the second wavelengthwhich has been transmitted through this region on the optical disc viathe object lens, the diffraction unit 150 which has the second and thirddiffraction regions 152 and 153 configured thus functions so as torestrict the numerical aperture of the optical beam of the secondwavelength to around NA=0.6. It should be noted however, that while inthis arrangement of the diffraction unit 150, the optical beam of thesecond wavelength is subjected to aperture restriction around NA=0.6,numerical aperture restriction due to the above configuration is notlimited to this.

Also, the third diffraction region 153 is formed of a size such that theoptical beam of the first wavelength which has been transmitted throughthe region thereof is input to the object lens 134 in the same state asan optical beam which has been subjected to aperture restriction ataround NA=0.85, and since there is no diffraction region formed on theouter side of the third diffraction region 153, this does not allowcondensation of the optical beam of the first wavelength which has beentransmitted through this region on the first optical disc via the objectlens, and the diffraction unit 150 which has the third diffractionregion 153 configured thus functions so as to restrict the numericalaperture of the optical beam of the first wavelength to around NA=0.85.Note that with the first wavelength optical beam transmitted through thethird diffraction region 153, light of diffraction of +1 order, +2order, +3 order, +4 order, and +5 order, for example is dominant, so thezero-order light transmitted through the region outside the thirddiffraction region 153 almost never passes through the object lens 134to be condensed on the first optical disc, but in cases wherein thiszero-order light does pass through the object lens 134 and is condensedon the first optical disc, a configuration may be provided to performaperture restriction by providing, at the region outside of the thirddiffraction region 153, either a shielding portion for shielding opticalbeams passing through, or a diffraction region having a diffractionstructure wherein optical beams of orders other than the order of theoptical beam passing through the object lens 134 to be condensed on thefirst optical disc are dominant. It should be noted however, that whilein this arrangement of the diffraction unit 150, the optical beam of thefirst wavelength is subjected to aperture restriction around NA=0.85,but the present invention is not restricted to this, i.e., numericalaperture restriction due to the above configuration is not limited tothis.

Specifically, as shown in FIGS. 22 and 23A, in the same way as with theabove-described first diffraction region 151, the third diffractionregion 153 is formed with the cross-sectional form of ring shapescentered on the optical axis being formed in a blazed shape having apredetermined depth d, for example.

Also, while description is made here with regard to the seconddiffraction region having the cross-sectional form of the rings formedas a diffraction structure with a blazed form, any diffraction structuremay be used as long as an optical beam of a predetermined order isdominant as to the optical beam of each wavelength as described above,so a configuration may be used such as shown in FIG. 23C, with adiffraction region 153B having a diffraction structure wherein thecross-sectional form of the rings is formed with the cross-sectionalform of ring shapes centered on the optical axis being formed in astaircase-like shape having a predetermined depth d and a predeterminednumber of steps S, continuing in the radial direction in a staircaseform, for example.

For example, the diffraction structure of the third diffraction region153B is, as shown in FIG. 23C, a diffraction structure having astaircase portion including first and second steps 153 s 1 and 153 s 2,formed continuously in the radial direction, wherein the number of stepsis 2 (S=2), and the depth of each step is generally the same depth(d/2), and first through third diffraction faces 153 f 1, 153 f 2, and153 f 3 formed at the same intervals of d/2 in the optical axisdirection.

Also, the third diffraction region 153 is configured such that thediffraction order k1 ois expressed with the following relation in a casewherein the k1 oorder diffracted light of the optical beam of the firstwavelength transmitted therethrough is dominant, i.e., so that thediffraction efficiency is maximum,1≦k1o≦5

where k1 ois a positive integer. That is to say, k1 ois one of k1 o=+1,+2, +3, +4, or +5.

Now, the third diffraction region 153 serving as the outer ring zone isselected by orders most excellent from the first through thirdperspectives described in the above description of the first diffractionregion 151, and accordingly, spherical aberration can be reduced,occurrence of aberration under temperature change can be reduced, aconfiguration which is advantageous in manufacturing can be had.

Now, as described above, the third diffraction region 153 is configuredso as to diffract light such that the diffraction efficiency of thediffracted light of the diffraction order k1 ofor the optical beam ofthe first wavelength passing through the object lens 134 is in a highstate, so as to form a suitable spot condensed on the signal recordingfaces of the first optical disc, and also to have an aperturerestriction function for suppressing the diffraction efficiency of thediffraction order of the optical beam of the second and thirdwavelengths to be condensed on the signal recording face of the secondand third optical discs as much as possible, but a configuration may bemade wherein the optical beam of this diffraction order in the opticalbeam of the second and third wavelengths are shifted from a statewherein the focal point is imaged on the signal recording face of thesecond and third optical discs, so as to further reduce the lightquantity of the optical beam substantially condensed on the signalrecording face of the second and third optical discs, i.e., whereflaring is employed.

Now, with regard to the third diffraction region 153, flaring and theconfiguration thereof will be described. Description has been made aboveregarding the first diffraction region 151 that there is the need tosatisfy the conditional expression of(λ1×k1x−λ2×k2x)/(t1−t2)≈(λ1×k1x−λ3×k3x)/(t1−t3),

this conditional expression (x=o for the outer ring zone in k1 x, k2 x,and k3 x in this conditional expression) being taken into considerationin the third diffraction region 153 as well. With regard to the thirddiffraction region 153 serving as the outer ring zone, giving thought tothe function of diffracting light such that the diffraction efficiencyof the diffracted light of the diffraction order k1 ofor the opticalbeams of the first wavelength passing through the object lens 134 is ina high state, so as to form a suitable spot condensed on the signalrecording faces of the first optical disc, as described above, the Pλ1to be plotted can be positioned on a design line, and further, a designline can be selected such that Pλ2 and Pλ3 corresponding to the secondand third wavelengths intentionally deviate from the design line, so asto cause flaring regarding the second wavelength or the thirdwavelength, or the second and third wavelengths.

That is to say, configuring the object lens 134 formed based on a designline wherein Pλ2 deviates from the design line allows the diffractedrays of the diffraction order of the optical beam of the secondwavelength to be shifted from a state of imaging the focal point on thesignal recording face of the second optical disc, so the quantity oflight of the optical beam of the second wavelength condensed on thesignal recording face of the second optical disc can be substantiallyreduced, and accordingly, aperture restriction regarding the opticalbeam of the second wavelength as described above can be performed in asure an excellent manner. Also, configuring the object lens 134 formedbased on a design line wherein Pλ3 deviates from the design line allowsthe diffracted rays of the diffraction order of the optical beam of thethird wavelength to be shifted from a state of imaging the focal pointon the signal recording face of the third optical disc, so the quantityof light of the optical beam of the third wavelength condensed on thesignal recording face of the third optical disc can be substantiallyreduced, and accordingly, aperture restriction regarding the opticalbeam of the third wavelength as described above can be performed in asure an excellent manner. Also, configuring the object lens 134 formedbased on a design line wherein both Pλ2 and Pλ3 deviate from the designline allows both of the above-described advantages to be had, i.e., thequantity of light of the optical beams of the second and thirdwavelengths condensed on the signal recording face of the correspondingoptical discs can be reduced.

Specifically, in the event that (k1 o, k2 o, k3 o)=(+1, +2, +2) asdescribed later with reference to FIG. 30, Pλ2 deviates from the designline L12, so in addition to the effect of reducing the diffractionefficiency of the diffracted light of the order of the second wavelengthdue to the diffraction structure formed on the third diffraction region153 which is an initially expected advantage, the advantages of flaringcan also be obtained, thereby enabling the quantity of light of theoptical beam of the second wavelength input to the second optical discto be further suppressed. Also, as described later with reference toFIG. 34, in the case of (k1 o, k2 o, k3 o)=(+4, +3, +3) both Pλ2 and Pλ3deviate from the design line L14, so in addition to the effect ofreducing the diffraction efficiency of the diffracted light of theorders of the second and third wavelengths due to the diffractionstructure formed on the third diffraction region 153 which is aninitially expected advantage, the advantages of flaring can also beobtained, thereby enabling the quantity of light of the optical beam ofthe second and third wavelengths input to the second and third opticaldiscs to be further suppressed.

Specific examples of the above-described diffraction unit 150 having thefirst diffraction region 151 which is the inner ring zone, seconddiffraction region 152 which is the middle ring zone, and thirddiffraction region 153 which is the outer ring zone, will be givenbelow, with specific numerical values of the depth d and number of stepsS in the staircase form or blazing, and the diffraction order ofdiffracted light of the order that is dominant in the optical beam ofeach wavelength, and the diffraction efficiency of the diffracted lightof each diffraction order is shown in Table 4 and the later-describedTable 5. Note that Table 4 illustrates a first embodiment of thediffraction unit 150 and Table 5 illustrates a second embodiment of thediffraction unit 150, wherein k1 in Tables 4 and 5 indicates thediffraction orders (k1 i, k1 m, k1 o) where the optical beam of thefirst wavelength is condensed at each ring zone via the object lens 134so as to form a suitable spot condensed on the signal recording face ofthe first optical disc, i.e., diffraction orders where diffractionefficiency is maximum, eff1 illustrates the diffraction efficiency ofthe diffraction orders (k1 i, k1 m, k1 o) for the optical beam of thefirst wavelength, k2 indicates the diffraction orders (k2 i, k2 m, k2 o)where the optical beam of the second wavelength is condensed via theobject lens 134 so as to form a suitable spot on the signal recordingface of the second optical disc, i.e., diffraction orders wherediffraction efficiency is maximum, particularly at the inner ring zoneand middle ring zone, eff2 illustrates the diffraction efficiency of thediffraction orders (k2 i, k2 m, k2 o) for the optical beam of the secondwavelength, k3 indicates the diffraction orders (k3 i, k3 m, k3 o) wherethe optical beam of the third wavelength is condensed via the objectlens 134 so as to form a suitable spot on the signal recording face ofthe third optical disc, i.e., diffraction orders where diffractionefficiency is maximum, particularly at the inner ring zone, eff3illustrates the diffraction efficiency of the diffraction orders (k3 i,k3 m, k3 o) for the optical beam of the third wavelength, d indicatesthe groove depth of each diffraction region, and S indicates the numberof steps in the case of the staircase form, with “∞” indicating a blazedshape. Note that the asterisks in Table 4 and Table 5 indicatediffraction order for condensing an optical beam passing through themiddle ring zone or the outer ring zone in each embodiment so as toappropriately form a spot on the signal recording face of thecorresponding optical disk via the object lens 134, i.e., a diffractionorder whereby spherical aberration on the signal recording face of thecorresponding optical disc can be corrected, or a diffraction order fora flared state as described later, and “≈0” indicates that thediffraction efficiency is at a state of approximately zero.

TABLE 4 Diffraction orders, diffraction efficiency, depth, and number ofsteps, of First Embodiment k1 eff₁ K2 eff₂ K3 eff₃ d [μm] s Inner Ringzone 1 0.91 1 0.73 1 0.53 0.9 ∞ Middle Ring zone 1 0.72 1 0.66 ※ ~0 5.13 Outer Ring zone 1 0.92 ※ ~0 ※ ~0 0.65 ∞

Now, the first embodiment shown in Table 4 will be described. At theinner ring zone in the first embodiment, as shown in Table 4, with ablazed form (S=∞) having a groove depth of d=0.9 (μm), the diffractionefficiency eff1=0.91 for the diffraction order k1 i=+1 of the opticalbeam of the first wavelength, the diffraction efficiency eff2=0.73 forthe diffraction order k2 i=+1 of the optical beam of the secondwavelength, and the diffraction efficiency eff3=0.53 for the diffractionorder k3 i=+1 of the optical beam of the third wavelength.

Next, the inner ring zone of the first embodiment will be described infurther detail with reference to FIGS. 27A through 27C. FIG. 27A is adiagram illustrating change in the diffraction efficiency of the +1order diffracted light of the optical beam of the first wavelength in acase of changing the groove depth d of the blazed form where the numberof steps S=∞, FIG. 27B is a diagram illustrating change in thediffraction efficiency of the +1 order diffracted light of the opticalbeam of the second wavelength in a case of changing the groove depth dof the blazed form where the number of steps S=∞, and FIG. 27C is adiagram illustrating change in the diffraction efficiency of the +1order diffracted light of the optical beam of the third wavelength in acase of changing the groove depth d of the blazed form where the numberof steps S=∞. In FIGS. 27A through 27C, the horizontal axis representsthe groove depth in nm, and the vertical axis represents the diffractionefficiency (intensity of light). As shown in FIG. 27A, at the positionof 900 nm on the horizontal axis, eff1 is 0.91, eff2 is 0.73 as shown inFIG. 27B, and eff3 is 0.53 as shown in FIG. 27C.

At the middle ring zone in the first embodiment, as shown in Table 4,with groove depth d=5.1 (μm) and the number of steps S=3, thediffraction efficiency eff1=0.72 for the diffraction order k1 m=+1 ofthe optical beam of the first wavelength, and the diffraction efficiencyeff2=0.66 for the diffraction order k2 m=+1 of the optical beam of thesecond wavelength. Also, the diffraction efficiency eff3 for thediffraction order k3 m(*) of the optical beam of the third wavelengthpassing through the region, for condensing light so as to form a spotwith the optical beam of the third wavelength on the signal recordingface of the third optical disc via the object lens 134 is approximatelyzero.

Next, the middle ring zone of the first embodiment will be described infurther detail with reference to FIGS. 28A through 28C. FIG. 28A is adiagram illustrating change in the diffraction efficiency of the +1order diffracted light of the optical beam of the first wavelength in acase of changing the groove depth d of the staircase form where thenumber of steps S=3, FIG. 28B is a diagram illustrating change in thediffraction efficiency of the +1 order diffracted light of the opticalbeam of the second wavelength in a case of changing the groove depth dof the staircase form where the number of steps S=3, and FIG. 28C is adiagram illustrating change in the diffraction efficiency of the +1order diffracted light of the optical beam of the third wavelength in acase of changing the groove depth d of the staircase form where thenumber of steps S=3. In FIGS. 28A through 28C, the horizontal axisrepresents the groove depth in nm, and the vertical axis represents thediffraction efficiency (intensity of light). As shown in FIG. 28A, atthe position of 5100 nm on the horizontal axis, eff1 is 0.72, eff2 is0.66 as shown in FIG. 28B, and eff3 is approximately zero as shown inFIG. 28C. Note that in Table 4 and the above, the diffraction order ofthe optical beam of the third wavelength noted with the asterisk “*” is+1.

Also, at the outer ring zone in the first embodiment, as shown in Table4, with a blazed form (S=∞) having a groove depth of d=0.6 (μm), thediffraction efficiency eff1=0.92 for the diffraction order k1 o=+1 ofthe optical beam of the first wavelength. Also, the diffractionefficiency eff2 for the diffraction order k2 o(*) of the optical beam ofthe second wavelength passing through the region, for condensing lightso as to form a spot on the signal recording face of the second opticaldisc via the object lens 134 is approximately zero, and the diffractionefficiency eff3 for the diffraction order k3 o(*) of the optical beam ofthe third wavelength passing through the region, for condensing light soas to form a spot on the signal recording face of the third optical discvia the object lens 134 is approximately zero.

Next, the outer ring zone of the first embodiment will be described infurther detail with reference to FIGS. 29A through 29C. FIG. 29A is adiagram illustrating change in the diffraction efficiency of the +1order diffracted light of the optical beam of the first wavelength in acase of changing the groove depth d of the blazed form where the numberof steps S=∞, FIG. 29B is a diagram illustrating change in thediffraction efficiency of the +2 order diffracted light of the opticalbeam of the second wavelength in a case of changing the groove depth dof the blazed form where the number of steps S=∞, and FIG. 29C is adiagram illustrating change in the diffraction efficiency of the +2order diffracted light of the optical beam of the third wavelength in acase of changing the groove depth d of the blazed form where the numberof steps S=∞. In FIGS. 29A through 29C, the horizontal axis representsthe groove depth in nm, and the vertical axis represents the diffractionefficiency (intensity of light). As shown in FIG. 29A, at the positionof 650 nm on the horizontal axis, eff1 is 0.92, eff2 is approximatelyzero as shown in FIG. 29B, and eff3 is approximately zero as shown inFIG. 29C. Note that in Table 4 and the above, the diffraction orders ofthe optical beams of the second and third wavelengths noted with theasterisk “*” are +2 and +2, respectively.

Also, with the outer ring zone in the first embodiment described above,of the design line in the relation between the above-described(wavelength×order) and the thickness of the protective layer, they-intercept position and inclination with the vertical axis representingthe thickness of the protective layer as the Y axis exhibits flaringregarding the second wavelength by change due to design of the objectlens. Accordingly, performing appropriate object lens design based onsuch a design line enables the quantity of light of the optical beam ofthe second wavelength to be further suppressed and excellent aperturerestriction to be performed regarding the optical beam of the secondwavelength. Specifically, as shown in FIG. 30, the outer ring zone inthe first embodiment has the design line indicated by L12 set byplotting the points Pλ1, Pλ2, and Pλ3 at the diffraction orders (k1 o,k2 o, k3 o)=(+1, +2, +2). In FIG. 30 the design point Pλ1 of the firstwavelength and the design point Pλ3 of the third wavelength arepositioned on the design line L12, so the aberration of diffractionlight of the diffraction orders k1 oand k3 ois approximately zero. Onthe other hand, the plotted point Pλ2 of the second wavelength issignificantly deviated from the aberration zero design point, indicatingthe above-described flaring. Note that in FIG. 30, only (k2 o, k3 o)=(2,2) is shown plotted, but there is deviation from the design line L12 inthe same way for other orders in the second and third wavelengths aswell. Consequently, there is uncorrected aberration in the secondwavelength, and consequently, the light quantity of the optical beam ofthe second wavelength which has passed through the outer ring zone, thatis not imaged at the signal recording face but input to the secondoptical disc can be suppressed. As a result, a suitable aperturerestriction (NA=0.6) can be realized, regardless of the diffractionefficiency of the second wavelength.

As described above, with the outer ring zone in the first embodiment,the diffraction face is blazed, so according to this configuration, evenin the case of providing the diffraction units to one face of the objectlens as described later, diffraction grooves can be formed relativelyeasily at the curved face of the lens face at the perimeter of the lenswhich has a steep slope due to being at the outer ring zone. Also, withthe outer ring zone in the first embodiment, the third wavelengthregarding which aperture restriction the same as with the secondwavelength is desired is condensed in a state of spherical aberrationhaving been corrected due to selecting the +2 order, but the diffractionefficiency is approximately zero as shown in FIG. 29C, whereby aperturerestriction functions can be manifested.

Next, description will be made regarding the second embodiment shown inTable 5. Note that the inner ring zone in the second embodiment is ofthe same configuration as that of the inner ring zone in the firstembodiment described above, as can be seen from Table 4 and Table 5, andaccordingly description thereof will be omitted.

TABLE 5 Diffraction orders, diffraction efficiency, depth, and number ofsteps, of Second Embodiment k1 eff₁ K2 eff₂ K3 eff₃ d [μm] s Inner Ringzone 1 0.91 1 0.73 1 0.53 0.9 ∞ Middle Ring zone 3 0.96 2 0.93 ※ ~0 2.4∞ Outer Ring zone 4 1.0 ※ ~0 ※ ~0 3.1 ∞

At the middle ring zone in the second embodiment, as shown in Table 5,with a blazed form (S=∞) having a groove depth of d=2.4 (μm), thediffraction efficiency eff1=0.96 for the diffraction order k1 m=+3 ofthe optical beam of the first wavelength, the diffraction efficiencyeff2=0.93 for the diffraction order k2 m=+2 of the optical beam of thesecond wavelength. Also, the diffraction efficiency eff3=0.48 for thediffraction order k3 m(*) of the optical beam of the third wavelengthpassing through the region, for condensing light so as to form a spot onthe signal recording face of the third optical disc via the object lens134, but as described later the spot is flared, and accordingly does notcontribute to imaging.

Next, the middle ring zone of the second embodiment will be described infurther detail with reference to FIGS. 31A through 31C. FIG. 31A is adiagram illustrating change in the diffraction efficiency of the +3order diffracted light of the optical beam of the first wavelength in acase of changing the groove depth d of the blazed form where the numberof steps S=∞, FIG. 31B is a diagram illustrating change in thediffraction efficiency of the +2 order diffracted light of the opticalbeam of the second wavelength in a case of changing the groove depth dof the blazed form where the number of steps S=∞, and FIG. 31C is adiagram illustrating change in the diffraction efficiency of the +2order diffracted light of the optical beam of the third wavelength in acase of changing the groove depth d of the blazed form where the numberof steps S=∞. In FIGS. 31A through 31C, the horizontal axis representsthe groove depth in nm, and the vertical axis represents the diffractionefficiency (intensity of light). As shown in FIG. 31A, at the positionof 2400 nm on the horizontal axis, eff1 is 0.96, eff2 is 0.93 as shownin FIG. 31B, and eff3 is 0.48 as shown in FIG. 31C, but the spot isflared, as described later. Note that here, the diffraction order of theoptical beam of the third wavelength noted with the asterisk “*” is +2in Table 5 and the above description.

Also, with the middle ring zone in the second embodiment, the designline of the object lens is changed for flaring of the third wavelength,thereby performing excellent aperture restriction, in the same way aswith the case of the outer ring zone in the first embodiment describedabove. Specifically, as shown in FIG. 33, the middle ring zone in thesecond embodiment has the design line indicated by L13 set by plottingthe points Pλ1, Pλ2, and Pλ3 at the diffraction orders (k1 m, k2 m, k3m)=(+3, +2, +2). In FIG. 33 the design point Pλ1 of the first wavelengthand the design point Pλ2 of the second wavelength are positioned on thedesign line L13, so the aberration of diffraction light of thediffraction orders k1 mand k2 mis approximately zero. On the other hand,the plotted point Pλ3 of the third wavelength is significantly deviatedfrom the aberration zero design point, indicating the above-describedflaring. Note that in FIG. 33, only k3 m=+2 is shown plotted, but thereis deviation from the design line L13 in the same way for other ordersin the third wavelength as well. Consequently, there is uncorrectedaberration in the third wavelength, and accordingly, the light quantityof the optical beam of the third wavelength which has passed through themiddle ring zone, that is not imaged at the signal recording face butinput to the third optical disc, can be suppressed. As a result, even ifthere is a little diffraction efficiency of the optical beam of thethird wavelength, as shown in FIG. 31C, this does not contribute to theimaging of these optical beams, and a suitable aperture restriction(NA=0.45) can be realized.

Also, the middle ring zone in the second embodiment described above hasa higher diffraction efficiency as to the first wavelength than themiddle ring zone in the first embodiment described above, and excels inthat perspective.

Also, at the outer ring zone in the second embodiment, as shown in Table5, with a blazed form (S=∞) having a groove depth of d=3.1 (μm), thediffraction efficiency eff1=1.0 for the diffraction order k1 o=+4 of theoptical beam of the first wavelength. Also, the diffraction efficiencyeff2=0.25 for the diffraction order k2 o(*) of the optical beam of thesecond wavelength, for condensing light so as to form a spot on thesignal recording face of the second optical disc via the object lens134, but as described later the spot is flared, and accordingly does notcontribute to imaging. Further, the diffraction efficiency eff3 for thediffraction order k3 o(*) of the optical beam of the third wavelengthpassing through the region, for condensing light so as to form a spot onthe signal recording face of the third optical disc via the object lens134 is approximately zero.

Next, the outer ring zone of the second embodiment will be described infurther detail with reference to FIGS. 32A through 32C. FIG. 32A is adiagram illustrating change in the diffraction efficiency of the +4order diffracted light of the optical beam of the first wavelength in acase of changing the groove depth d of the blazed form where the numberof steps S=∞, FIG. 32B is a diagram illustrating change in thediffraction efficiency of the +3 order diffracted light of the opticalbeam of the second wavelength in a case of changing the groove depth dof the blazed form where the number of steps S=∞, and FIG. 32C is adiagram illustrating change in the diffraction efficiency of the +3order diffracted light of the optical beam of the third wavelength in acase of changing the groove depth d of the blazed form where the numberof steps S=∞. In FIGS. 32A through 32C, the horizontal axis representsthe groove depth in nm, and the vertical axis represents the diffractionefficiency (intensity of light). As shown in FIG. 32A, at the positionof 3100 nm on the horizontal axis, eff1 is 1.0, eff2 is 0.25 as shown inFIG. 32B, but the spot is flared as will be described later. Further,eff3 is approximately zero as shown in FIG. 32C. Note that in Table 5and the above, the diffraction orders of the optical beams of the secondand third wavelengths noted with the asterisk “*” are +3 and +3,respectively.

Also, with the outer ring zone in the second embodiment, the design lineof the object lens is changed for flaring of the second and thirdwavelengths, thereby performing excellent aperture restriction, in thesame way as with the case of the outer ring zone in the first embodimentdescribed above. Specifically, as shown in FIG. 34, the outer ring zonein the second embodiment has the design line indicated by L14 set byplotting the points Pλ1, Pλ2, and Pλ3 at the diffraction orders (k1 o,k2 o, k3 o)=(+4, +3, +3). In FIG. 34 the design point Pλ1 of the firstwavelength and is positioned on the design line L14, so the aberrationof diffracted light of the diffraction order k1 ois approximately zero.On the other hand, the plotted points Pλ2 and Pλ3 of the second andthird wavelengths are significantly deviated from the aberration zerodesign point, indicating the above-described flaring. Note that in FIG.34, only (k2 o, k3 o)=(+3, +3) is shown plotted, but there is deviationfrom the design line L14 in the same way for other orders in the secondand third wavelengths as well. Consequently, there is uncorrectedaberration in the second and third wavelengths, and accordingly, thelight quantity of the optical beams of the second and third wavelengthswhich has passed through the outer ring zone, that is not imaged at thesignal recording face but input to the second and third optical discscan be suppressed. As a result, even if there is a little diffractionefficiency of the optical beam of the second wavelength as shown in FIG.32, this does not contribute to the imaging of these optical beams, anda suitable aperture restriction (NA=0.6) can be realized. Also, an evenmore suitable aperture restriction (NA=0.45) can be realized for theoptical beam of the third wavelength.

Now, while it can be said that the outer ring zone in the firstembodiment is basically easier to employ from a design perspective,there is demand for reduced aberration change due to temperature asdescribed above in the diffraction unit having such an outer ring zoneas described above, and the outer ring zone in the second embodiment isadvantageous from this aspect. This will be described using theabove-described effect term ΔWn due to refractive index fluctuation ofthe composition material under change in temperature, and effect termΔWλ due to wavelength fluctuation of the incident optical beam underchange in temperature. Generally, |ΔWn| is greater than |ΔWλ|, so it isdifficult to realize ΔW≈0 with a diffraction order around 1 or so. Also,the effect term ΔWλ is generally proportionate to the diffraction order,so employing as great a diffraction order as possible can increase theΔWλ which can be understood as being aberration change amount occurringdue to diffraction, thereby aiming to realize ΔW≈0 of the sphericalaberration ΔW due to temperature rise. A design example according tothis perspective is the outer ring zone (k1 o=+4) according to thesecond embodiment described with reference to FIGS. 32A through 32C andFIG. 34, and the amount of aberration occurring at the time oftemperature change can be reduced as compared to the outer ring zoneaccording to the first embodiment where k1 o=+1 is employed. Describingthis with a longitudinal aberration diagram in the same way as with FIG.25 above, if we say that a longitudinal aberration diagram accompanyingthe temperature change in a case of (k1 i, k1 m, k1 o)=(+1, +1, +1) isobtained as in FIG. 25B, in a case of selecting relatively high orderdiffraction orders at the middle ring zone and the outer ring zone suchthat (k1 i, k1 m, k1 o)=(+1, +3, +4), a state such as shown in FIG. 25Cis obtained. In FIG. 25C, the dotted line Lwn is the same as in FIG.25B, the single-dot broken line Lwλ3 represents the effect term theeffect term ΔWλ due to wavelength fluctuation in the case of selecting arelatively high order diffraction order for the middle ring zone andouter ring zone, as longitudinal aberration. In FIG. 25C the solid lineLw3 represents spherical aberration ΔW occurring due to temperaturerise, obtained by adding the effect term ΔWn and effect term ΔWλindicated by Lwn and Lwλ3. Thus, it can be seen from FIG. 25C thatoccurrence of longitudinal aberration (Lw3) is further suppressed ascompared to the longitudinal aberration amount shown by the solid lineLw2 in FIG. 25B.

With the diffraction unit of the second embodiment having such an innerring zone, middle ring zone, and outer ring zone, diffraction efficiencyas to the first wavelength in particular is excellent for all ringzones, thereby realizing high diffraction efficiency as to the firstwavelength, for which there has been strong demand regardingthree-wavelength compatibility but which has been difficult withcompatibility lenses which have been studied with relation to therelated art.

The diffraction unit 150 and the object lens 134, having the firstthrough third diffraction regions 151, 152, and 153 with theconfiguration such as described above, are capable of condensation ofthe optical beams of the first through third wavelengths passing throughthe first diffraction region 151 so as to form a suitable spot on thesignal recording face of the corresponding optical disc by being inputto the object lens 134, in a divergent angle state wherein no sphericalaberration occurs at the signal recording face of respectivelycorresponding optical discs via the common object lens 34, i.e., in aconverged state wherein spherical aberration is corrected via the objectlens 134, and is capable of condensation of the optical beams of thefirst and second wavelengths passing through the second diffractionregion 152 so as to form a suitable spot on the signal recording face ofthe corresponding optical disc by being input to the object lens 134, ina divergent angle state wherein no spherical aberration occurs at thesignal recording face of respectively corresponding optical discs viathe common object lens 34, i.e., in a converged state wherein sphericalaberration is corrected via the object lens 134, and also is capable ofcondensation of the optical beams of the first wavelength passingthrough the third diffraction region 153 so as to form a suitable spoton the signal recording face of the corresponding optical disc by beinginput to the object lens 134, in a divergent angle state wherein nospherical aberration occurs at the signal recording face of thecorresponding optical disc via the object lens 34, i.e., in a dispersedstate or converged state wherein spherical aberration is corrected viathe object lens 134.

That is to say, the diffraction unit 150 provided on one face of thediffraction optical element 135 disposed on the optical path between thefirst through third emitting units in the optical system of the opticalpickup 103 and the signal recording face allows optical beams ofrespective wavelengths passing through the respective regions (firstthrough third diffraction regions 151, 152, and 153) to be input to theobject lens 134 in a state wherein spherical aberration occurring at thesignal recording face to be reduced, so spherical aberration occurringat the signal recording face when condensing optical beams of the firstthrough third wavelengths on the signal recording face of the respectivecorresponding optical discs using the common object lens 134 in theoptical pickup 3 can be minimized, which is to say that three-wavelengthcompatibility of the optical pickup 3 using three types of wavelengthsfor three types of optical discs and a common object lens 134 can berealized, wherein information signals can be recorded to and/or playedfrom respective optical discs.

Also, the above-described diffraction unit 150 and object lens 134having the first through third diffraction regions 151, 152, and 153,are configured such that the diffraction orders (k1 i, k2 i, k3 i) oflight selected by the first diffraction region 151 serving as the innerring zone and condensed on the signal recording face of thecorresponding optical disk via the object lens 134 are set to (+1, +1,+1), light can be condensed on the signal recording face of each opticaldisc in a state of the three wavelengths having spherical aberrationreduced and with a high diffraction efficiency for each, i.e., withsufficient light quantity, and also, spherical aberration occurring dueto change in temperature reduced, and further, the groove depth of thediffraction structure to be formed can be prevented from becoming toodeep so manufacturing is easily, and the problem of deterioration inprecision and so forth is prevented, thereby obtaining a configurationwhich is advantageous from the perspective of manufacturing.

Further, the diffraction unit 150 and object lens 134 are configuredsuch that the diffraction orders (k1 m, k2 m) of light selected by thesecond diffraction region 152 serving as the middle ring zone andcondensed on the signal recording face of the corresponding optical diskvia the object lens 134 are set to (+1, +1) or (+3, +2), light can becondensed on the signal recording face of each optical disc in a stateof the first and second wavelengths having spherical aberration reducedand with sufficient light quantity, and also, spherical aberrationoccurring due to change in temperature reduced, thereby obtaining aconfiguration which is advantageous from the perspective ofmanufacturing, and further, advantages of the above-described flaringcan be obtained as well.

Moreover, the diffraction unit 150 and object lens 134 are configuredsuch that the diffraction order k1 oof light selected by the thirddiffraction region 153 serving as the outer ring zone and condensed onthe signal recording face of the corresponding optical disk via theobject lens 134 is set to +1, +2, +3, +4, +5, so light can be condensedon the signal recording face of each optical disc in a state of thefirst wavelength having spherical aberration reduced and with sufficientlight quantity, and also, spherical aberration occurring due to changein temperature reduced, thereby obtaining a configuration which isadvantageous from the perspective of manufacturing, and further,advantages of the above-described flaring can be obtained as well.

Also, the diffraction unit 150 having the first through thirddiffraction regions 151, 152, and 153 is capable of suitably solving theproblems of diffraction efficiency and spherical aberration at the timeof temperature change, of which solving has been difficult withthree-wavelength compatible lenses studied with relation to the relatedart. That is, with the three-wavelength compatible lenses studied withrelation to the related art, raising the design efficiency of the firstwavelength which is the shortest wavelength has been difficult, andfurther the curvature at the lens perimeter is great due to being athree-wavelength compatible lens so there has been the problem such asnecessary diffraction efficiency not being able to be obtained whendiffraction efficiency drops due to the precision in form of thediffraction structure formed at the perimeter portion being low, and theproblem that even if aberration can be suppressed when diffractionorders of opposite signs are selected for the first through thirdwavelengths, aberration increases for wavelengths regarding whichdiffraction orders of opposite signs are selected, due to inversion inbehavior at the time of temperature changing between diffraction ordersof opposite signs being selected for the first through thirdwavelengths, and that generally with such diffraction units, the amountof spherical aberration occurring due to the refraction index at thetime of temperature rising is cancelled out by the amount of sphericalaberration occurring due to wavelength fluctuation at the time oftemperature rising, and that the sign of effect of the amount ofspherical aberration occurring due to wavelength fluctuation at the timeof temperature rising is determined by the diffraction direction;however, with the above-described diffraction unit 150 having the firstthrough third diffraction regions 151, 152, and 153, the designefficiency as to the first wavelength can be raised to almost 100%, andalso occurrence of spherical aberration at the time of temperaturechange can be suppressed.

Further, by forming the first diffraction region 151 of the diffractionunit 150 with a blazed form having a shallow groove depth to realizethree-wavelength compatibility, the manufacturing processing becomeseasy, enabling simplification of manufacturing and reduction in costs,and particularly, the case of integrating the diffraction unit with theobject lens as described later, a configuration advantageous from theperspective of manufacturing can be obtained. Also, by forming thesecond and third diffraction regions 152 and 153 of the diffraction unit150 with a blazed form having a shallow groove depth, the manufacturingprocessing becomes easy, enabling simplification of manufacturing andreduction in costs, and particularly, the case of integrating thediffraction unit with the object lens as described later, aconfiguration advantageous from the perspective of manufacturing can beobtained.

Also, the diffraction unit 150 having the first through thirddiffraction regions 151, 152, and 153 is configured such that an orderother than the diffraction order, whereby the optical beam of the thirdwavelength passing through the second and third diffraction regions 152and 153 is suitably condensed on the signal recording face of thecorresponding type of optical disc via the object lens 134, is dominant,so that only the portion of the optical beam which has passed throughthe first diffraction region 151 is condensed on the signal recordingface of the optical disc via the object lens 134, and the firstdiffraction region 151 is formed to a size such that the optical beam ofthe third wavelength passing through this region is shaped to have asize of a predetermined numerical aperture, whereby aperture restrictioncan be performed regarding the optical beam of the third wavelength soas to have a numerical aperture of around 0.45, for example. Note thatby forming the diffraction unit 150 and object lens 134 such thatflaring is implemented regarding the third wavelength as described atone or both of the second and third diffraction regions 152 and 153,whereby the light quantity of the optical beam of third wavelengthcondensed on the signal recording face of the third optical disc isfurther suppressed, thereby enabling manifesting of further aperturerestriction functions.

Also, the diffraction unit 150 is configured such that an order otherthan the diffraction order, whereby the optical beam of the secondwavelength passing through the third diffraction region 153 is suitablycondensed on the signal recording face of the corresponding type ofoptical disc via the object lens 134, is dominant, so that only theportion of the optical beam which has passed through the first andsecond diffraction regions 151 and 152 is condensed on the signalrecording face of the optical disc via the object lens 134, and thefirst and second diffraction regions 151 and 152 are formed to a sizesuch that the optical beam of the second wavelength passing through thisregion is shaped to have a size of a predetermined numerical aperture,whereby aperture restriction can be performed regarding the optical beamof the second wavelength so as to have a numerical aperture of around0.60, for example. Note that by forming the diffraction region 150 andobject lens 134 such that flaring is implemented regarding the secondwavelength as described at the third diffraction region 153, whereby thelight quantity of the optical beam of the second wavelength condensed onthe signal recording face of the second optical disc is furthersuppressed, thereby enabling manifesting of further aperture restrictionfunctions.

Also, the diffraction unit 150 performs places the optical beam of thefirst wavelength passing outside of the third diffraction region 153 ina state so as to not be suitably condensed on the signal recording faceof the corresponding type of optical disc via the object lens 134, orshields the optical beam of the first wavelength passing outside of thethird diffraction region 153, whereby, with regard to the optical beamof the first wavelength, only the optical beam portion which has passedthrough the first through third diffraction regions 151, 152, and 153 iscondensed on the signal recording face of the optical disc via theobject lens 134, and also, the first through third diffraction regions151, 152, and 153 are formed to a size which is the numerical apertureof the first wavelength optical beam passing through this region,whereby aperture restriction can be performed regarding the optical beamof the first wavelength such that NA=around 0.85, for example.

Thus, the diffraction unit 150 provided on one face of the diffractionoptical element 135 disposed on the optical path as described above notonly realizes three-wavelength compatibility, but also enables opticalbeams of each wavelength to be input to the common object lens 134 in astate wherein aperture restriction is performed appropriately with anumerical aperture corresponding to each of the three types of opticaldiscs and optical beams of the first through third wavelengths. Thus,the diffraction unit 150 not only has functions of aberration correctioncorresponding to the three wavelengths, but also has functions as anaperture restricting unit.

It should be noted that a diffraction unit can be configured by suitablycombining the diffraction regions in the above-described embodiments.That is to say, the diffraction order of each wavelength passing througheach diffraction region can be selected as appropriate. In the event ofchanging the diffraction order of each wavelength passing through eachdiffraction region, an object lens 134 corresponding to each diffractionorder of each wavelength passing through each region can be used.

Also, while description has been made above with the diffraction unit150 configured of the three diffraction regions 151, 152, and 153 formedon the incident side face of the diffraction optical element 135provided separately from the object lens 134, as shown in FIG. 35A, thepresent invention is not restricted to this arrangement, and may beprovided to the output side face of the diffraction optical element 135.Further, the diffraction unit 150 having the first through thirddiffraction regions 151, 152, and 153, can be integrally configured onthe input or output side face of the object lens 134, or, as shown inFIG. 35B for example, an object lens 134B having the diffraction unit150 on the incident side face thereof may be configured. In the event ofproviding the diffraction unit 150 on the incident side face of theobject lens 134B for example, the planar shape of the above-describeddiffraction structure is combined with a reference face at the incidentside face required for the lens to be able to function as an objectlens. While the above-described diffraction optical element 135 and theobject lens 134 are two separate elements serving as a condensingoptical device, the object lens 134B thus configured functions as acondensing optical device which can perform suitable light condensingsuch that spherical aberration does not occur at the signal recordingface of optical discs corresponding to each of the three optical beamsof different wavelengths, with a single element. Providing thediffraction unit 150 integrally with the object lens 134B enablesfurther reduction in optical parts and also reduction in configurationsize. The object lens 134B having a diffraction unit having functionsthe same as the diffraction unit 150 provided integrally at the inputside or output side face realizes three-wavelength compatibility of theoptical pickup by reducing aberration and so forth when used in anoptical pickup, and also reduces the number of parts so as to enablesimplification and reduction in size of the configuration, therebyrealizing high production and reduced costs. Note that theabove-described diffraction unit 150 sufficiently manifests theadvantages thereof with the diffraction structure for aberrationcorrection to realize three-wavelength compatibility being provided on asingle face that has been difficult with the related art, which enablessuch a diffraction element to be integrally formed with the object lens134 serving as such a refractive element, further enabling directlyforming a diffraction face on a plastic lens, and forming the objectlens 134B with which the diffraction unit 150 has been integrated of aplastic material further realizing improved production and lower costs.

The collimator lens 142 provided between the diffraction optical element135 and the third beam splitter 138 converts the divergent angle of eachof the first through third wavelength optical beams of which the opticalpaths have been synthesized at the second beam splitter 137 and passedthrough the third beam splitter 138, and outputs to the quarter-waveplate 143 and diffraction optical element 135 side, in a generallyparallel light state, for example. The arrangement wherein thecollimator lens 142 inputs the optical beams of the first and secondwavelengths into the above-described diffraction optical element 135with the divergent angle thereof in the state of generally parallellight, and also inputs the optical beam of the third wavelength into thediffraction optical element 135 with divergent angle in a state which isslightly diffused as to parallel light (hereinafter also referred to as“finite system state”) enables further reduction of sphericalaberration, slightly occurring at the time of condensing the thirdwavelength optical beam on the signal recording face of the thirdoptical disc via the diffraction optical element 135 and the object lens134, described with reference to FIG. 24, to realize three-wavelengthcompatibility with less aberration occurring. While an arrangement hasbeen described here wherein the optical beam of the third wavelength isinput to the diffraction optical element 135 in a state of apredetermined divergent angle, due to the positional relation betweenthe third light source 133 having the third emitting unit for emittingthe third wavelength optical beam and the collimator lens 142, in theevent of positioning multiple emitting units at a common light sourcefor example, this may be realized by providing an element which convertsonly the divergent angle of the optical beam of the third wavelength, orby inputting into the diffraction optical element 135 in a predetermineddivergent angle state by providing a mechanism to drive the collimatorlens 142, or the like. Also, the optical beam of the second wavelength,or the optical beams of the second and third wavelengths, may be inputto the diffraction optical element 135 in the finite system state,thereby further reducing aberration. Also, optical beams of the secondand third wavelengths may be input in the finite system state and in adiffused state, thereby realizing adjustment of return power and evenmore excellent optical system compatibility may be achieved by settingthe focus capture range and so forth to a desired state matching theformat by adjusting the return power. Note that in this case, the objectlens 134 is formed with the design line situated downwards by apredetermined distance with regard to the plotted points Pλ2 and Pλ3with regard to the second and third wavelengths in the relation betweenthe wavelength×diffraction order and protective layer thicknessdescribed above.

The multi-lens 146 is, for example, a wavelength-selective multi-lens,whereby the returning first through third wavelength optical beamsseparated from the outgoing path optical beams by being reflected at thethird beam splitter 138, after having been reflected off of the signalrecording face of the respective optical disc, and passed through theobject lens 134, diffraction optical element 135, redirecting mirror144, quarter-wave plate 143, and collimator lens 142, is appropriatelycondensed on the photoreception face of the photodetector or the like ofthe photosensor 145. At this time, the multi-lens 146 provides thereturn optical beam with astigmatism for detection of focus errorsignals or the like.

The photosensor 145 receives the return optical beam condensed at themulti-lens 146, and detects, along with information signals, varioustypes of detection signals such as focus error signals, tracking errorsignals, and so forth.

With the optical pickup 103 configured as described above, the objectlens 134 is driven so as to be displaced based on the focus errorsignals and tracking error signals obtained by the photosensor 145,whereby the object lens 134 is moved to a focal position as to thesignal recording face of the optical disc 2, the optical beam is focusedonto the signal recording face of the optical disc 2, and information isrecorded to or played from the optical disc 2.

The optical pickup 103 is provided on one face of the diffractionoptical element 135, can provide optical beams of each wavelength with adiffraction efficiency and diffraction angle suitable for each regiondue to the diffraction unit 150 having the first through thirddiffraction regions 151, 152, and 153, can sufficiently reduce sphericalaberration at the signal recording face of the three types of firstthrough third optical discs 11, 12, and 13, of which the format for thethickness of the protective layer or the like differs, and enablesreading and writing of signals to and from the multiple types of opticaldiscs 11, 12, and 13, using optical beams of three differentwavelengths.

Also, the diffraction optical element 135 having the diffraction unit150, and object lens 134, in the above optical pickup 103, can functionas a condensing optical device for condensing incident optical beams ata predetermined position. In the event of using an optical pickup whichperforms recording and/or playing of information signals by irradiatingoptical beams onto three different types of optical discs, thediffraction unit 150 provided on one face of the diffraction opticalelement 135 enables the condensing optical device to appropriatelycondense corresponding optical beams onto the signal recording face ofthe three types of optical discs in a state with spherical aberrationsufficiently reduced, meaning that three-wavelength compatibility of theoptical pickup using the object lens 134 common to the three wavelengthscan be realized.

Also, while description has been made above regarding a configurationwherein the diffraction optical element 135 to which the diffractionunit 150 is provided, and the object lens 134, are provided to anactuator such as an object lens driving mechanism or the like fordriving the object lens 134 is as to be integral, this may be configuredas a condensing optical unit wherein the diffraction optical element 135and the object lens 134 are formed as an integrated unit, in order toimprove precision of assembly to the lens holder of the actuator, andfacilitate assembly work. For example, a condensing optical unit can beconfigured by use spacers or the like to fix the diffraction opticalelement 135 and object lens 134 to the holder while setting thepositioning, spacing, and optical axis, so as to be integrally formed.Due to being integrally assembled to the object lens driving mechanismas described above, the diffraction optical element 135 and object lens134 can appropriately condense the first through third wavelengthoptical beams on the signal recording face of the respective opticaldiscs in a state with spherical aberration reduced, even at the time offield shift such as displacement in the tracking direction.

Next, the optical paths of the optical beams emitted from the firstthrough third light sources 131, 132, and 133 of the optical pickup 103configured as described above, will be described with reference to FIG.2. First, the optical path at the time of emitting the optical beam ofthe first wavelength as to the first optical disc 11 and performingreading or writing of information will be described.

The disc type determination unit 22 which has determined that the typeof the optical disc 2 is the first optical disc 11 causes the opticalbeam of the first wavelength to be emitted from the first emitting unitof the first light source 131.

The optical beam of the first wavelength emitted from the first emittingunit is split into three beams by the first grating 139, for detectionof tracking error signals and so forth, and is input to the second beamsplitter 137. The optical beam of the first wavelength which has beeninput to the second beam splitter 137 is reflected at a mirror face 137a thereof, and is output to the third beam splitter 138 side.

The optical beam of the first wavelength which is input to the thirdbeam splitter 138 is transmitted through a mirror face 138 a thereof,output to the collimator lens 142 side, where the divergent angle isconverted so as to be generally parallel light by the collimator lens142, provided with a predetermined phase difference at the quarter-waveplate 143, reflected off of the redirecting mirror 144, and output tothe diffraction optical element 135 side.

The optical beam of the first wavelength which is input to thediffraction optical element 135 is output with the optical beam whichhas passed through each region thereof having a predetermineddiffraction order (k1 i, k1 m, k1 o) dominant therein as describedabove, due to the first through third diffraction regions 151, 152, and153 of the diffraction unit 150 provided on the incident side facethereof, and input to the object lens 134. The optical beam of the firstwavelength output from the diffraction optical element 135 is not onlyin a state of a predetermined divergent angle, but also is in a state ofaperture restriction.

The optical beam of the first wavelength input to the object lens 134has been input in a converged state of the divergent angle wherebyspherical aberration of the optical beam having passed through theregions 151, 152, and 153 can be reduced, and accordingly isappropriately condensed by the object lens 134 on the signal recordingface of the first optical disc 11.

The optical beam condensed at the first optical disc 11 is reflected atthe signal recording face, passes through the object lens 134,diffraction optical element 135, redirecting mirror 144, quarter-waveplate 143, and collimator lens 142, is reflected off of the mirror face138 a of the third beam splitter 138, and is output to the photosensor145 side.

The optical beam split from the optical path of the outgoing opticalbeam reflected off of the third beam splitter 138 is condensed on thephotoreception face of the photosensor by the multi-lens 146, anddetected.

Next, description will be made regarding the optical path at the time ofemitting an optical beam of the second wavelength to the second opticaldisc 12 and reading or writing information. The disc type determinationunit 22 which has determined that the type of the optical disc 2 is thesecond optical disc 12 causes the optical beam of the second wavelengthto be emitted from the second emitting unit of the second light source132.

The optical beam of the second wavelength emitted form the secondemitting unit is split into three beams by the second grating 140, fordetection of tracking error signals and so forth, and is input to thefirst beam splitter 136. The optical beam of the second wavelength whichhas been input to the first beam splitter 136 is transmitted through amirror face 136 a thereof, also transmitted through the mirror face 137a of the second beam splitter 137, and is output to the third beamsplitter 138 side.

The optical beam of the second wavelength which is input to the thirdbeam splitter 138 is transmitted through the mirror face 138 a thereof,output to the collimator lens 142 side, where the divergent angle isconverted so as to be generally parallel light or diffused light, by thecollimator lens 142, provided with a predetermined phase difference atthe quarter-wave plate 143, reflected off of the redirecting mirror 144,and output to the diffraction optical element 135 side.

The optical beam of the second wavelength which is input to thediffraction optical element 135 is output with the optical beam whichhas passed through each region thereof having a predetermineddiffraction order dominant therein as described above, due to the firstthrough third diffraction regions 151, 152, and 153 of the diffractionunit 150 provided on the incident side face thereof, and input to theobject lens 134. The optical beam of the second wavelength output fromthe diffraction optical element 135 is not only in a state of apredetermined divergent angle, but also is in a state of aperturerestriction due to being input to the object lens 134.

The optical beam of the second wavelength input to the object lens 134has been input in a divergent angle state whereby spherical aberrationof the optical beam having passed through the first and seconddiffraction regions 151 and 152 can be reduced, and accordingly isappropriately condensed by the object lens 134 on the signal recordingface of the second optical disc 12.

The return side optical path of the optical beam reflected off of thesignal recording face of the second optical disc 12 is the same as withthe case of the above-described optical beam of the first wavelength,and accordingly description thereof will be omitted.

Next, description will be made regarding the optical path at the time ofemitting an optical beam of the third wavelength to the third opticaldisc 13 and reading or writing information. The disc type determinationunit 22 which has determined that the type of the optical disc 2 is thethird optical disc 13 causes the optical beam of the third wavelength tobe emitted from the third emitting unit of the third light source 133.

The optical beam of the third wavelength emitted from the third emittingunit is split into three beams by the third grating 141, for detectionof tracking error signals and so forth, and is input to the first beamsplitter 136. The optical beam of the third wavelength which has beeninput to the first beam splitter 136 is reflected off of the mirror face136 a thereof, transmitted through the mirror face 137 a of the secondbeam splitter 137, and is output to the third beam splitter 138 side.

The optical beam of the third wavelength which is input to the thirdbeam splitter 138 is transmitted through the mirror face 138 a thereof,output to the collimator lens 142 side, where the divergent angle isconverted by the collimator lens 142 so as to be diffused as togenerally parallel light, provided with a predetermined phase differenceat the quarter-wave plate 143, reflected off of the redirecting mirror144, and output to the diffraction optical element 135 side.

The optical beam of the third wavelength which is input to thediffraction optical element 135 is output with the optical beam whichhas passed through each region thereof having a predetermineddiffraction order dominant therein as described above, due to the firstthrough third diffraction regions 151, 152, and 153 of the diffractionunit 150 provided on the incident side face thereof, and input to theobject lens 134. The optical beam of the third wavelength output fromthe diffraction optical element 135 is not only in a state of apredetermined divergent angle, but also is in a state of aperturerestriction due to having been input to the object lens 134.

The optical beam of the third wavelength input to the object lens 134has been input in a divergent angle state whereby spherical aberrationof the optical beam having passed through the first diffraction region151 can be reduced, and accordingly is appropriately condensed by theobject lens 134 on the signal recording face of the third optical disc13.

The return side optical path of the optical beam reflected off of thesignal recording face of the third optical disc 13 is the same as withthe case of the above-described optical beam of the first wavelength,and accordingly description thereof will be omitted.

Note that while a configuration has been described here wherein theoptical beam of the third wavelength has the position of the thirdemitting unit adjusted such that the optical beam of which the divergentangle is converted by the collimator lens 142 and input to thediffraction optical element 135 is in a diffused state as to a state ofgenerally parallel light, a configuration may be made wherein theoptical beam is input to the diffraction optical element 135 byproviding an element which has wavelength selectivity and converts thedivergent angle, or by providing a mechanism which drives the collimatorlens 142 in the optical axis direction.

Also, while description has been made regarding a configuration whereinthe optical beam of the first wavelength is input to the diffractionoptical element 135 in a state of generally parallel light, the opticalbeam of the second wavelength is input to the diffraction opticalelement 135 in a state of generally parallel light or diffused light,and the optical beam of the third wavelength is input to the diffractionoptical element 135 in a diffused state, the present invention is notrestricted to this arrangement, and configurations may be made wherein,for example, the first through third wavelength optical beams areselectively input to the diffraction optical element 135 in a state ofdiffused light, parallel light, or converged light, taking intoconsideration the diffraction order selected according to the designline of the object lens 134 and diffraction unit 150.

The optical pickup 103 to which the present invention has been appliedhas first through third emitting units for emitting optical beams offirst through third wavelengths, an object lens 134 for condensing theoptical beams of first through third wavelengths emitted from the firstthrough third emitting units into a signal recording face of an opticaldisc, and a diffraction unit 150 provided on one face of an opticalelement disposed on the outgoing optical path of the optical beams offirst through third wavelengths, wherein the diffraction unit 150 hasfirst through third diffraction regions 151, 152, and 153, with thefirst through third diffraction regions 151, 152, and 153 beingdifferent diffraction structures circular in shape and having apredetermined depth, and the first through third diffraction structureswhereby optical beams of each wavelength are diffracted such thatdiffracted light of a predetermined diffraction order is dominant asdescribed above, and according to this configuration, optical beamscorresponding to each of three types of optical discs having differenceusage wavelengths can be appropriately condensed on the signal recordingface using the common object lens 134, thereby realizing excellentrecording and/or playing of information signals to/from the respectiveoptical discs by realizing three-wavelength compatibility with thecommon object lens 134, without necessitating a complex structure.

That is to say, the optical pickup 103 to which the present inventionhas been applied obtains optimal diffraction efficiencies anddiffraction angels for the first through third wavelength optical beamsdue to the diffraction unit 150 provided on one face within the opticalpath thereof, whereby signals can be read from and written to themultiple types of optical discs 11, 12, and 13, using the optical beamsof different wavelengths emitted from the multiple emitting unitsprovided to each of the light sources 131, 132, and 133, and alsooptical parts such as the object lens 134 and so forth can be shared,thereby reducing the number of parts, simplifying and reducing the sizeof the configuration, and realizing high production and lower costs.

The optical pickup 103 to which the present invention has been appliedis configured with the diffraction unit 150 and object lens 134 has thepredetermined diffraction orders (k1 i, k2 i, k3 i) selected by thefirst diffraction region 151 set to (+1, +1, +1), whereby light can becondensed on the signal recording face of each optical disc withsufficiently high light use efficiency while reducing sphericalaberration to the three wavelengths, and also excellent sphericalaberration properties at the time of temperature change can be obtained,thereby realizing excellent compatibility and realizing excellentrecording and/or playing to/from each optical disc.

The optical pickup 103 to which the present invention has been appliedis configured with the diffraction unit 150 and object lens 134 has thepredetermined diffraction orders (k1 m, k2 m) selected by the secondand/or third diffraction regions 152 and 153 set to (+1, +1) or (+3, +2)and k1 oset to +1, +2, +3, +4, and +5, whereby light can be condensed onthe signal recording face of each optical disc with sufficiently highlight use efficiency while reducing spherical aberration to thecorresponding wavelengths, and with particularly high light useefficiency regarding the optical beam of the first wavelength, and alsoeven more excellent spherical aberration properties at the time oftemperature change can be obtained, thereby realizing even moreexcellent compatibility and realizing excellent recording and/or playingto/from each optical disc.

Also, the optical pickup 103 to which the present invention has beenapplied can share the object lens 134 between the three wavelengths,thereby preventing trouble of reduction of sensitivity of the actuatorand so forth due to increase weight of moving parts. Also, the opticalpickup 103 to which the present invention has been applied cansufficiently reduce spherical aberration which is problematic in thecase of sharing the object lens 134 between the three wavelengths, dueto the diffraction unit 150 provided on one face of the optical element,so problems such as positioning of diffraction units one to another inthe event that multiple diffraction units are provided on multiple facesto reduce spherical aberration as with the related art, anddeterioration of diffraction efficiency due to providing of the multiplediffraction units, can be prevented, which realizes simplification ofthe assembly process and improved usage efficiency of light. Also, withthe optical pickup 103 to which the present invention has been applied,a configuration wherein the diffraction unit 150 is provided on one faceof the optical element as described above enables a configuration havingan object lens 134B including the diffraction unit 150 instead of theobject lens 134 and the diffraction optical element 135, and byintegrally forming the diffraction unit 150 with the object lens,realizes further simplification of the structure, reduction in weight ofmoving parts of the actuator, simplification of the assembly process,and improved usage efficiency of light.

Further, the optical pickup 103 to which the present invention has beenapplied not only realizes three-wavelength compatibility with thediffraction unit 150 provided on the one face of the diffraction opticalelement described above, but also can perform aperture restriction witha numerical aperture corresponding to each of the three types of opticaldiscs and optical beams of three types, thereby doing away with the needfor aperture restriction filters or the like which have been necessarywith the related art, and also adjustment in the positioning thereof,which enables further simplification of configuration, reduction insize, and reduction in costs. Also, the optical pickup 103 has aconfiguration wherein the above-described flaring is enabled at one orboth of the second and third diffraction regions 152 and 153 at thediffraction unit 150 and object lens 134, thereby manifesting even moreexcellent aperture restriction functions.

Also, while the above optical pickup 103 has been described having thefirst emitting unit provided at the first light source 131, the secondemitting unit provided at the second light source 132, and the thirdemitting unit provided at the third light source 133, the presentinvention is not restricted to this arrangement, and an arrangement maybe made wherein a light source having two of the first through thirdemitting units, and another light source having the remaining oneemitting unit, are provided at different positions.

Next, description will be made regarding an optical pickup 160 shown inFIG. 36 including a light source having a first emitting unit, and alight source having second and third emitting units. Note that portionsin the following description which are the same as with the opticalpickup 103 will be denoted with the same reference numerals, anddescription thereof will be omitted.

As shown in FIG. 36, the optical pickup 160 to which the presentinvention has been applied includes a first light source 161 having afirst emitting unit for emitting an optical beam of a first wavelength,a second light source 162 having a second emitting unit for emitting anoptical beam of a second wavelength and a third emitting unit foremitting an optical beam of a third wavelength, an object lens 134 forcondensing optical beams emitted from the first through third emittingunits onto the signal recording face of an optical disc 2, and adiffraction optical element 135 provided on the optical path between thefirst through third emitting units and the object lens 134. Thisdiffraction optical element 135 is provided with the diffraction unit150, as described above. Also, with the optical pickup 160 describedhere as well, a configuration may be made wherein the diffraction unit150 is integrally provided on one face of the optical lens, either theinput side or output side, such as with the above-described object lens134B for example, instead of the object lens 134 and the diffractionoptical element 135.

Also, the optical pickup 160 includes a beam splitter 163 serving as anoptical path synthesizing unit for synthesizing the optical paths of theoptical beam of the first wavelength that has been emitted from thefirst emitting unit of the first light source 161 and the optical beamsof the second and third wavelengths that have been emitted from thesecond and third emitting unit of the second light source 162, and abeam splitter 164 serving the same function as the above third beamsplitter 138.

Further, the optical pickup 160 has a first grating 139, and a grating165 with wavelength dependency, provided between the second light sourceunit 162 and the beam splitter 163, for diffracting the optical beams ofthe second and third wavelengths that have been emitted from the secondand third emitting units into three beams, for detection of trackingerror signals and so forth.

Also, the optical pickup 160 has a collimator lens 142, quarter-waveplate 143, redirecting mirror 144, photosensor 145, and multi-lens 146,and also a collimator lens driving unit 166 for driving the collimatorlens 142 in the direction of the optical axis. The collimator lensdriving unit 166 can adjust the divergent angle of optical beams passingthrough the collimator lens 142 as described above by driving thecollimator lens 142 in the direction of the optical axis, whereby notonly can spherical aberration be reduced by inputting the optical beamsto the diffraction optical element 135 and object lens 134 in a desiredstate, but in the event that the mounted optical disc is a so-calledmulti-layer optical disc having multiple signal recording faces,recording and/or playing to/from each of the signal recording faces isenabled.

With the optical pickup 160 configured as described above, the functionsof each of the optical parts is the same as with the optical pickup 103except for those mentioned above, and the optical paths of the opticalbeams of the first through third wavelengths emitted from the firstthrough third emitting units are the same as with the optical pickup 103except for the above-mentioned, i.e., following synthesizing of theoptical paths of the optical beams of each wavelength by the beamsplitter 164, so detailed description thereof will be omitted.

The optical pickup 160 to which the present invention has been appliedhas first through third emitting units for emitting optical beams offirst through third wavelengths, an object lens 134 for condensing theoptical beams of first through third wavelengths emitted from the firstthrough third emitting units into a signal recording face of an opticaldisc, and a diffraction unit 150 provided on one face of an opticalelement disposed on the outgoing optical path of the optical beams offirst through third wavelengths, wherein the diffraction unit 150 hasfirst through third diffraction regions 151, 152, and 153, with thefirst through third diffraction regions 151, 152, and 153 beingdifferent diffraction structures circular in shape and having apredetermined depth, and the first through third diffraction structureswhereby optical beams of each wavelength are diffracted such thatdiffracted light of a predetermined diffraction order is dominant asdescribed above, and according to this configuration, optical beamscorresponding to each of three types of optical discs having differentusage wavelengths can be appropriately condensed on the signal recordingface using the single shared object lens 134, thereby realizingexcellent recording and/or playing of information signals to/from therespective optical discs by realizing three-wavelength compatibilitywith the common object lens 134, without necessitating a complexstructure. The optical pickup 160 also has the other advantages of theabove-described optical pickup 103, as well.

Further, the optical pickup 160 is configured such that the second andthird emitting units are positioned at a common light source 162,thereby realizing further simplification of configuration and reductionin size. Note that in the same way, with the optical pickup to which thepresent invention has been applied, the first through third emittingunits may be positioned at a light source at generally the sameposition, thereby realizing further simplification of configuration andreduction in size with such a configuration.

The optical disc device 1 to which the present invention has beenapplied has a driving unit for holding and rotationally driving anoptical disc arbitrarily selected from the first through third opticaldiscs, and an optical pickup for performing recording and/or playing ofinformation signals from/to the optical disc being rotationally drivenby the driving unit by selectively irradiating one of multiple opticalbeams of different wavelengths corresponding to the optical disc, and byusing the above-described optical pickups 103 or 160 as the opticalpickup, optical beams corresponding to each of three types of opticaldiscs having different usage wavelengths can be appropriately condensedon the signal recording face due to the diffraction unit provided on oneface of the optical element on the optical path of the optical beams ofthe first through third wavelengths, using a single common object lens134, thereby realizing excellent recording and/or playing of informationsignals to/from the respective optical discs by realizingthree-wavelength compatibility with the common object lens 134, whileenabling simplification of the configuration and reduction in size,without necessitating a complex structure.

<4 > Third Embodiment of Optical Pickup (FIGS. 37 through 59)

Next, an optical pickup 203 to which the present invention is appliedwill be described in detail as a third embodiment of the optical pickupused in the above-described optical disc device 1, with reference toFIGS. 37 through 59. As described above, the optical pickup 203 is anoptical pickup which selectively irradiates multiple optical beams ontooptical discs selected from first through third optical discs 11, 12,and 13, of which the format such as the thickness of the protectivelayer differs, thereby performing recording and/or playing ofinformation signals.

Note that the optical pickup 203 serving as the third embodimentdescribed here is for solving the same problems as those of theabove-mentioned optical pickups 3 and 103, and additionally, solving thefollowing problems, and includes an arrangement for obtaining moreadvantageous effects. Firstly, with the optical pickup 203, demands forrealizing enhancement of light use efficiency can be handled, and aproblem for reducing focal distance as to the first wavelength whileholding a suitable operating distance of the third wavelength can besolved, and with regard to these points, the optical pickup 203 excelsthe above-mentioned optical pickup 3. Secondly, with the optical pickup203, demands for reducing unwanted light incidence can be handled, and aproblem for optimizing operating distance and focal distance can besolved by changing the order of diffraction selected for the first andthird wavelengths, and with regard to these points, the optical pickup203 excels the above-mentioned optical pickup 103.

As shown in FIG. 37, the optical pickup 203 to which the presentinvention has been applied includes a first light source 231 having afirst emitting unit for emitting an optical beam of a first wavelength,a second light source 232 having a second emitting unit for emitting anoptical beam of a second wavelength which is longer than the firstwavelength, a third light source 233 having a third emitting unit foremitting an optical beam of a third wavelength which is longer than thesecond wavelength, an object lens 234 which serves as a condensingoptical device for condensing optical beams emitted from the firstthrough third emitting units onto the signal recording face of anoptical disc 2.

Also, the optical pickup 203 includes a first beam splitter 236 providedbetween the second and third emitting units and the object lens 234,serving as an optical path synthesizing unit for synthesizing theoptical paths of the optical beam of the second wavelength that has beenemitted from the second emitting unit and the optical beam of the thirdwavelength that has been emitted from the third emitting unit, a secondbeam splitter 237 provided between the first beam splitter 236 and theobject lens 234, serving as an optical path synthesizing unit forsynthesizing the optical paths of the optical beams of the second andthird wavelengths of which the optical paths have been synthesized bythe first beam splitter 236, and the optical path of the optical beam ofthe first wavelength that has been emitted from the first emitting unit,and a third beam splitter 238 provided between the second beam splitter237 and the object lens 234, serving as an optical path splitting unitfor splitting the outgoing optical path of the optical beam of the firstthrough third wavelengths of which the optical paths have beensynthesized at the second beam splitter 237 from the returning opticalpath of the optical beam of the first through third wavelengthsreflected at the optical disc (hereinafter also referred to as “returnpath”).

Further, the optical pickup 203 has a first grating 239 provided betweenthe first emitting unit of the first light source unit 231 and thesecond beam splitter 237, for diffracting the optical beam of the firstwavelength that has been emitted from the first emitting unit into threebeams, for detection of tracking error signals and so forth, a secondgrating 240 provided between the second emitting unit of the secondlight source unit 232 and the first beam splitter 236, for diffractingthe optical beam of the second wavelength that has been emitted from thesecond emitting unit into three beams, for detection of tracking errorsignals and so forth, and a third grating 241 provided between the thirdemitting unit of the third light source unit 233 and the first beamsplitter 236, for diffracting the optical beam of the third wavelengththat has been emitted from the third emitting unit into three beams, fordetection of tracking error signals and so forth.

Also, the optical pickup 203 has a collimator lens 242 provided betweenthe third beam splitter 238 and the object lens 234, serving as adivergent angle conversion unit for converting the divergent angle ofthe optical beam of the first through third wavelength of which theoptical paths have been synthesized at the third beam splitter 238 so asto be adjusted into a state of generally parallel light or a statediffused or converged as to generally parallel light, and outputting, aquarter-wave plate 243 provided between the collimator lens 242 and theobject lens 234, so as to provide quarter-wave phase difference to theoptical beam of the first through third wavelength of which thedivergent angle has been adjusted, and a redirecting mirror 244 providedbetween the object lens 234 and the quarter-wave plate 243, forredirecting the optical beam which has passed through theabove-described optical parts within a plane generally orthogonal to theoptical axis of the object lens 234, so as to emit the optical beam inthe optical axis direction of the object lens 234.

Further, the optical pickup 203 includes a photosensor 245 for receivingand detecting the optical beams of the first through third wavelengthssplit at the third beam splitter 238 on the return path from the opticalbeam of the first through third wavelengths on the outgoing path, and amulti lens 246 provided between the third beam splitter 238 and thephotosensor 245, for condensing optical beams of the first through thirdwavelengths split at the third beam splitter 238 onto the photoreceptionface of a photodetector or the like of the photosensor 245, and alsoproviding astigmatism for detecting focus error signals or the like.

The first light source 231 has a first emitting unit for emitting anoptical beam of a first wavelength around 405 nm onto the first opticaldisc 11. The second light source 232 has a second emitting unit foremitting an optical beam of a second wavelength around 655 nm onto thesecond optical disc 12. The third light source 233 has a third emittingunit for emitting an optical beam of a third wavelength around 785 nmonto the third optical disc 13. Note that while the first through thirdemitting units are configured disposed at individual light sources 231,232, and 233, the invention is not restricted to this, and anarrangement may be made wherein two emitting units of the first throughthird emitting units are disposed at one light source and the remainingemitting unit is disposed at another light source, or wherein the firstthrough third emitting units are disposed so as to form a light sourceat generally the same position.

The object lens 234 condenses the input optical beams of the firstthrough third wavelengths into the signal recording face of the opticaldisc 2. The object lens 234 is movably held by an object lens drivingmechanism such as an unshown biaxial actuator or the like. The objectlens 234 is driven along two axes, one in the direction toward/away fromthe optical disc 2, and the other in the radial direction of the opticaldisc 2, by being moved by a biaxial actuator or the like based on thetracking error signals and focus error signals generated from the FRsignals of the return light from the optical disc 2 that has beendetected at the photosensor 245. The object lens 234 condenses opticalbeams emitted from the first through third emitting units such that theoptical beams are always focused on the signal recording face of opticaldisc 2, and also causes the focused beam to track a recording trackformed on the signal recording face of the optical disc 2. Note that anarrangement is made wherein, as described later, in the case of adiffraction unit 250 being provided on an optical element (diffractionoptical element 235B) separate from the object lens (see FIG. 58), thelater-described diffraction optical element 235B is held by a lensholder of the object lens driving mechanism where the object lens 234Bis held so as to be integral with the object lens 234B enables thelater-described advantages of the diffraction unit 250 provided to thediffraction optical element 235B to be suitably manifested at the timeof field shift of the object lens 234B such as movement in the trackingdirection.

Also, with the object lens 234, as one face thereof, for example, thediffraction unit 250 made up of multiple diffraction regions is providedon the incident side face, and according to this diffraction unit 250,each of the optical beams of the first through third wavelengths passingthrough each of the multiple diffraction regions is diffracted so as tobecome a predetermined order, thereby entering the object lens 234 asoptical beams in a diffused state or converged state having apredetermined divergent angle, and accordingly, the single object lens234 can be used to perform suitable condensing of the optical beams ofthe first through third wavelengths such that spherical aberration doesnot occur at the signal recording face of the three types of opticaldiscs corresponding to the optical beams of the first through thirdwavelengths. The object lens 234 including the diffraction unit 250serves as a condensation optical device for appropriately performingcondensation such that no spherical aberration occurs at the signalrecording face of the three types of optical discs corresponding to theoptical beams of the three different wavelengths by a diffractionstructure being formed which generates diffraction power with a lensface shape for generating diffraction power serving as reference. Also,thus, the object lens 234 has both of a refraction element function anda diffraction element function, i.e., has both of a refraction functionaccording to a lens curved surface, and a diffraction function accordingto the diffraction unit 250 provided on one face.

Now, in order to describe the diffraction function of the diffractionunit 250 conceptually, as described later, description will be maderegarding a case wherein the diffraction unit 250 is provided on thediffraction optical element 235B separate from the object lens 234Bhaving refractive power (see FIG. 58) as an example. The diffractionoptical element 235B, which is employed along with the object lens 234Bhaving a refraction function alone as described later, having thediffraction unit 250 performs, for example, as shown in FIG. 38A,diffraction of the first wavelength optical beam BB0 which hastransmitted the diffraction unit 250 so as to become +1st orderdiffracted beam BB1 and inputs to the object lens 234B, i.e., as a beamin a diffused state having a predetermined divergent angle, therebyappropriately condensing on the signal recording face of the firstoptical disc 11, as shown in FIG. 38B, performs diffraction of thesecond wavelength optical beam BD0 which has transmitted the diffractionunit 250 so as to become −1st order diffracted beam BD1 and inputs tothe object lens 234B, i.e., as a beam in a converged state having apredetermined divergent angle, thereby appropriately condensing on thesignal recording face of the second optical disc 12, as shown in FIG.38C, and performs diffraction of the third wavelength optical beam BC0which has transmitted the diffraction unit 250 so as to become −2ndorder diffracted beam BC1 and inputs to the object lens 234B, i.e., as abeam in a converged state having a predetermined divergent angle,thereby appropriately condensing on the signal recording face of thethird optical disc 13, whereby suitable condensation can be performedsuch that no spherical aberration occurs at the signal recording face ofthe three types of optical discs, with a single object lens 234B. Whiledescription has been made here with an example wherein optical beams ofthe same wavelength are made to be diffracted beams of the samediffraction order at the multiple diffraction regions of the diffractionunit 250, with reference to FIG. 38, the diffraction unit 250configuring the optical pickup 3 to which the present invention isapplied enables diffraction order corresponding to each wavelength to beset for each region as described later, so as to perform suitableaperture restriction, and further reduce spherical aberration.Description has been made so far regarding a case wherein thediffraction unit 250 is provided on an optical element separate from theobject lens for the sake of description as an example, but thediffraction unit 250 provided integral with one face of the object lens234 described here also has the same function by providing diffractionpower according to the diffraction structure thereof, and thediffraction power of the diffraction unit 250, and the refractive poweraccording to a lens curved face serving as the reference of the objectlens 234 enable the optical beams of each wavelength to be condensedappropriately on the signal recording face of the corresponding opticaldisc such that no spherical aberration occurs.

In the above and following description of diffraction orders, an orderof diffraction which draws closer to the optical axis side in thedirection of progression with regard to an input optical beam is apositive order, and an order of diffraction which separates from theoptical axis in the direction of progression is a negative order. Inother words, an order which diffracts toward the optical axis of theinput optical beam is a positive order.

Specifically, as shown in FIGS. 39A and 39B, the diffraction unit 250provided at the incident side face of the object lens 234 has agenerally-circular first diffraction region 251 provided on theinnermost portion (hereinafter also referred to as “inner ring zone”), aring-shaped second diffraction region 252 provided on the outer side ofthe first diffraction region 251 (hereinafter also referred to as“middle ring zone”), and a ring-shaped third diffraction region 253provided on the outer side of the second diffraction region 252(hereinafter also referred to as “outer ring zone”).

The first diffraction region 251 which is an inner ring zone has a firstdiffraction structure formed having a ring shape with a predetermineddepth, and diffracts the optical beam of the first wavelength that istransmitted therethrough such that diffracted light of an order whichforms an appropriate spot on the signal recording face of the firstoptical disc via the object lens 234 is dominant, i.e., such thatmaximum diffraction efficiency is manifested regarding diffracted lightof other orders.

Also, the first diffraction region 251 diffracts the optical beam of thesecond wavelength that is transmitted therethrough such that diffractedlight of an order which forms an appropriate spot on the signalrecording face of the second optical disc via the object lens 234 isdominant, i.e., such that maximum diffraction efficiency is manifestedregarding diffracted light of other orders, by way of the firstdiffraction structure.

The first diffraction region 251 diffracts the optical beam of the thirdwavelength that is transmitted therethrough such that diffracted lightof an order which forms an appropriate spot on the signal recording faceof the third optical disc via the object lens 234 is dominant, i.e.,such that maximum diffraction efficiency is manifested regardingdiffracted light of other orders, by way of the first diffractionstructure.

Thus, the first diffraction region 251 has a diffraction structureformed whereby diffracted light of a predetermined order is dominant inthe optical beam of each wavelength, thereby enabling correction andreduction of spherical aberration at the time of optical beams of eachwavelength that have passed through the first diffraction region 251 andbecome diffracted light of a predetermined order being condensed on thesignal recording face of the respective optical discs by the object lens234.

Note that regarding the first diffraction region 251, and also thesecond and third diffraction regions 252 and 253 described in detaillater, description is made in the above and below with the understandingthat transmitted light, i.e., light of zero order, is included in thediffracted light of a predetermined order selected so as to be dominantwith regard to the optical beam of each wavelength.

Specifically, as shown in FIGS. 39 and 40A, the first diffraction region251 is formed with the cross-sectional form of ring shapes being formedas to the reference face of ring shapes centered on the optical axis,and with step shapes (hereafter referred to as “multiple steps of stepshapes”) of a predetermined number of steps S (S is assumed to be apositive integer) of a predetermined depth (hereinafter also referred toas “groove depth”) d being formed consecutively in the radial direction.Note that the cross-sectional form of the ring shapes in thisdiffraction structure means the cross-sectional form of the rings takenalong a plane including the radial direction of the rings, i.e., a planeorthogonal to the tangential direction of the rings.

Also, this reference face means the face shape of the incident side facerequired as a refraction element function of the object lens 234. Withthe first diffraction region 251, in reality, as shown in FIG. 39A, withthe face shape of the incident side face required as a refractionelement function of the object lens 234 as a reference face, as to thisreference face, there is formed a face shape such as a combination of aring zone form face shape and staircase form face shape making up adiffraction structure having a diffraction function such as shown inFIG. 40A, but in FIG. 39A through 39C and later-described FIG. 47, adiffraction structure shape alone as to the reference face thereof isillustrated for the sake of description, and also in the followingdescription, the shape as to the reference face will be described. Notethat in the case of providing the diffraction unit 250 in an opticalelement (later-described diffraction optical element 235B) separate fromthe object lens, the shapes illustrated in FIGS. 39A through 39C becomethe cross-sectional shape of the relevant diffraction optical element235B. Also, the diffraction structure illustrated in FIG. 39 and soforth is actually formed with minute dimensions such as described later,and FIG. 39 and so forth illustrate enlarged cross sections.

Also, the diffraction structure having the staircase form with apredetermined number of steps S is a structure in which a staircase formhaving first through S steps, each of which have generally the samedepth, continuing in the radial direction, which can be rephrased assaying that the structure has first through S+1'th diffraction facesformed with generally the same interval in the optical axis direction.Also, the predetermined depth d in the diffraction structure means thelength along the optical axis between the diffraction face of the S+1'thdiffraction face which is formed at the side of the staircase formclosest to the surface (i.e., the highest step, which is the shallowestposition) and diffraction face of the first diffraction face which isformed at the side of the staircase form closest to the optical element(i.e., the lowest step, which is the deepest position). Note that whilea structure has been illustrated in FIG. 40A wherein the steps of eachstepped portion of the staircase shape are formed such that the closerto the inner side in the radial direction, the closer to the surfaceside the steps are formed, this is because a later-described diffractionorder is selected as the maximum diffraction efficiency order in aninner ring zone. Also, in FIGS. 40B and 40C, and later-described FIG.47, examples are illustrated wherein similar to an inner ring zone, thesaw-tooth slopes of the protrusions and recesses or the stepped portionsof the staircase shape are formed such that the closer to the inner sidein the radial direction, the closer to the surface side the saw-toothslopes of the protrusions and recesses or the stepped portions of thestaircase shape are formed, the present invention is not restricted tothis, the formation direction of the blazed shape or staircase shape isset according to the selected diffraction order. Ro in FIG. 40A through40C indicates the direction toward the outer side in the radialdirection of a ring zone, i.e., the direction separated from the opticalaxis.

In the first diffraction structure and the later-described second andthird diffraction structures formed at the first diffraction region 251,the groove depth d and number of steps S are determined taking intoconsideration the dominant diffraction order and diffraction efficiency.Also, as shown in FIG. 40A, the groove width of each step portion (theradial-direction dimension of each step portion of the staircase form)is such that the steps are formed with equal width within one staircaseform, while looking at the different staircase forms formed continuouslyin the radial direction, the value of the step width is smaller asstaircase forms further away from the optical axis. Note thatdescription has been made here assuming that such an arrangement isemployed as described above, but the groove width of each step portionis such that while looking at the different staircase forms formedcontinuously in the radial direction, the value of the step width isgrater as staircase forms further away from the optical axis in somecases. This point is also true for FIGS. 40B and 40C. Note that thegroove widths are determined based on phase difference obtained at thediffraction regions formed with the groove widths, such that the spotcondensed on the signal recording face of the optical disc is optimal.

For example, the diffraction structure of the first diffraction region251 is, as shown in FIG. 40A, a diffraction structure having a staircaseportion including first through fourth steps 251 s 1, 251 s 2, 251 s 3,and 251 s 4, formed continuously in the radial direction, wherein thenumber of steps is 4 (S=4), and the depth of each step is generally thesame depth (d/4), and first through fifth diffraction faces 251 f 1, 251f 2, 251 f 3, 251 f 4, and 251 f 5 formed at the same intervals of d/4in the optical axis direction.

Also, in a case wherein the first diffraction region 251 diffracts theoptical beam of the first wavelength which is transmitted therethroughsuch that diffracted light of the k1 i'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, diffracts theoptical beam of the second wavelength which is transmitted therethroughsuch that diffracted light of the k2 i'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, and diffracts theoptical beam of the third wavelength which is transmitted therethroughsuch that diffracted light of the k3 i'th order is dominant, anarrangement is made so as to have the relation of k1 i≧k2 i>k3 i.

Thus, according to the arrangement wherein diffracted light is generatedso as to have the relation of k1 i≧k2 i>k3 i, the first diffractionregion 251 makes not only the diffracted light of an order wherebyspherical aberration can be reduced appropriately dominant but also therelation between operating distance and focal distance changed to mostappropriate state, ensuring the operating distance in the case ofemploying the third wavelength λ3 makes the focal distance long as tothe first wavelength λ1, whereby problems can be prevented such as thelens diameter of the object lens and the optical pickup overallincreasing in size, and also aberration can be reduced while ensuringdiffraction efficiency.

Now, description will be made regarding a method for selecting theoptimal diffraction order including the reason why an arrangement ismade so as to have the relation of k1 i≧k2 i≧k3 i with the firstdiffraction region 251 based on the following first through fourthperspectives. In other words, with the first diffraction region 251, asthe first perspective there is a need to reduce spherical aberration ateach wavelength, as the second perspective there is a need to optimizeoperating distance and focal distance at each wavelength, and as thethird and fourth perspectives there is a need to employ the structurewhich is advantageous in manufacturing and can be readily manufactured,and consequently, from these perspectives, the diffraction orders k1 i,k2 i, and k3 i have been selected as diffraction orders with maximumdiffraction efficiency, and description will be made below regardingthis point.

First, the first perspective will be described. As the firstperspective, there is a need to employ an order whereby the sphericalaberration of the corresponding optical disc can be corrected at thetime of condensing light with the object lens 234 as the diffractionorder with the first diffraction region 251 which is an inner ring zone.In general, in a case wherein material dispersion is ignored at a regionhaving a function such as the first diffraction region 251, it is knownthat satisfying the conditional expression(λ1×k1x−λ2×k2x)/(t1−t2)≈(λ1×k1x−λ3×k3x)/(t1−t3)  (1)

where

-   -   λ1 is the first wavelength (nm),    -   λ2 is the second wavelength (nm),    -   λ3 is the third wavelength (nm),    -   k1 i is the diffraction order where an optical beam of the first        wavelength is selected,    -   k2 i is the diffraction order where an optical beam of the        second wavelength is selected,    -   k3 i is the diffraction order where an optical beam of the third        wavelength is selected,    -   t1 is the thickness (mm) of the first protective layer of the        first optical disc,    -   t2 is the thickness (mm) of the first protective layer of the        second optical disc,    -   t3 is the thickness (mm) of the first protective layer of the        third optical disc, and    -   x=i for the inner ring zone in k1 x, k2 x, and k3 x in this        conditional expression,

is a condition whereby spherical aberration on the signal recording faceof each optical disc at each wavelength can be corrected and reduced.

In the first diffraction region 251 which is the above-described innerring zone, when λ1=405 (nm), λ2=655 (nm), λ3=785 (nm), t1=0.1 (mm),t2=0.6 (mm), and t3=1.1 (mm), then k1 i=+1, k2 i=−1, and k3 i=−2, eachhold, thereby satisfying the conditional expression, and it has beenconfirmed that spherical aberration can be reduced. This can be restatedin other words that when plotting points Pλ1, Pλ2, and Pλ3 in the graphin FIG. 41 wherein the horizontal axis represents a value calculated bywavelength×diffraction order (nm) and the vertical axis represents thethickness (mm) of the protective layer, the points are on a generallystraight design line, meaning that spherical aberration on the signalrecording face of each optical disc at each wavelength can be correctedand reduced, but actually when plotting the respective points Pλ1, Pλ2,and Pλ3 under the following conditions, the respective points arepositioned on a generally straight design line, meaning that sphericalaberration can be corrected and reduced. Specifically, the object lens234 has the material of which it is configured, and the face shape atthe input and output sides, determined with the line L21 in FIG. 41 asthe design line, with the inclination of the design line approximatingthe inclination of the line connecting Pλ1 and Pλ2 calculated by(t1−t2)/(λ1×k1 i−λ2×k2 i) or the inclination of the line connecting Pλ1and Pλ3 calculated by (t1−t3)/(λ1×k1 i−λ3×k3 i), or determined takinginto consideration the inclination of these lines and other designconditions.

Note that while in FIG. 41 Pλ3 deviates slightly upwards from the lineL21, spherical aberration can be corrected in a sure manner by inputtingthe incident light to the object lens 234 where the diffraction unit 250is provided, as a divergent ray. That is to say, a divergent ray isinput to the object lens 234, whereby the same result as that in thecase of the apparent thickness of the protective layer being thickenedcan be obtained. Note that, as described later, in the case of providingthe diffraction unit 250 in an optical element (diffraction opticalelement 235B, see FIG. 58) separate from the object lens, sphericalaberration can be corrected in a sure manner by inputting the incidentlight to the one of the object lens 234B and diffraction optical element235B which is closer to the emitting units, which is, for example, thediffraction optical element 235 in FIG. 58, as a divergent ray.

Description will be made regarding this point with reference to FIG. 42illustrating the concept of this correction. Specifically, the opticalbeams of the second and third wavelengths λ2 and λ3 are input to theobject lens 234 as minimal divergent rays, thereby shifting plots Pλ2′and Pλ3′ indicating the second and third wavelengths upward as to theplots Pλ2 and Pλ3 according to the apparent thickness of the protectivelayer, as shown in FIG. 42. As shown in FIG. 42, the magnification of adivergent ray is adjusted appropriately, whereby these three points Pλ1,Pλ2′, and Pλ3′ can be positioned on one straight line L21′ completely,and spherical aberration due to difference of protective layer thicknessand so forth can be fully corrected. At this time, the straight lineL21′ where the plots Pλ1, Pλ2′, and Pλ3′ are positioned are taken as adesign line.

Note here that, for example, an arrangement may be made wherein only theoptical beam of the third wavelength λ3 is input as a convergent ray,and is shifted downward to position the respective plots on a straightline, thereby correcting spherical aberration, but employing aconvergent ray shortens the operating distance, which is undesirable insome cases, and accordingly, it is desirable to employ a divergent rayas described above. Further, when compatibility of three wavelengths istaken into consideration, it is advantageous to input a divergent ray tothe object lens with the second and third wavelengths from theperspective wherein appropriate return magnification can be ensured.

Also, when the plots Pλ1, Pλ2, and Pλ3 having close connection with theabove-mentioned relational expression, described with reference to FIG.41 are taken into consideration, if the absolute values of therespective orders k1 i, k2 i, and k3 i are within a range of around 3rdorder, there is a need to satisfy the following relational expression(2A) or (2B).k1i≦k2i≦k3i  (2A)k1i≧k2i≧k3i  (2B)

Next, the second perspective will be described. As the secondperspective, there is a need to employ an order whereby focal distancef1 as to the first wavelength λ1 can be reduced while maintaining theoperating distance WD3 large in the case of employing the thirdwavelength λ3. In general, extending the focal distance f extends theoperating distance. The focal distance f1 as to the first wavelength λ1needs to be reduced, and the focal distance f3 as to the thirdwavelength λ3 needs to be increased. Now, it is desirable to suppressthe focal distance f1 as to the first wavelength λ1 to 2.2 mm orshorter. Also, there is a need to ensure the operating distance ofaround 0.4 mm or longer in the case of employing the third wavelengthλ3. In order to realize these, if we say that f1=2.2 mm, and incidenceto the object lens 234 is infinite incidence, i.e., parallel lightincidence, f3 needs to be around 2.5 mm or longer. With the material ofthe object lens made from plastics corresponding to the above-mentionedthree wavelengths λ1, λ2, and λ3, dispersion is great, but let us saythat this is ignored here, and an approximate value is calculated.

The object lens 234 has refractive power according to a lens curvedface, and diffraction power according to the diffraction unit 250provided on one face. It has been known that focal distance F_(dif)according to diffraction of the diffraction unit 250 of the object lens234 can be calculated in accordance with the following Expression (3).In Expression (3), λ0 is a manufacturing wavelength, and now, let us saythat λ0=λ1. Also, C₁ is a value called a phase difference functioncoefficient, which is a coefficient for stipulating a phase differenceshape provided by a diffraction structure (diffraction grating), and isa variable value depending on the value of λ0. Also, in Expression (3),k represents a diffraction order selected by each of the wavelengths λ1,λ2, and λ3, and specifically is k1, k2, or k3.

$\begin{matrix}{f_{dif} = {\frac{0.5}{k\; C_{1}} \cdot \frac{\lambda_{0}}{\lambda}}} & (3)\end{matrix}$

In Expression (3), with the coefficient C₁, if we say that λ0=λ1, theabsolute value thereof is not smaller than 1×10⁻², the amount of pitchesincreases, and consequently, formation becomes impossible. Also, if wesay that the focal distance according to the refractive power of a lenscurved face is fr, focal distance f_(all) of the refraction anddiffraction overall of the object lens is calculated according to therelation of Expression (4) using the above-mentioned focal distancef_(dif) according to diffraction, and this fr.

$\begin{matrix}{\frac{1}{f_{all}} = {\frac{1}{f_{dif}} + \frac{1}{f_{r}}}} & (4)\end{matrix}$

FIG. 43 illustrates change in the value of the focal distance f3 whenchanging k1 and k3 based on such Expressions (3) and (4). In FIG. 43,the horizontal axis represents the order k3, and the vertical axisrepresents the focal distance f3 as to the third wavelength λ3, andcurves LM3, LM2, LM1, LP0, LP1, LP2, and LP3 represent curves connectingplotted changes in the focal distance f3 along with change in k3 i inthe case of the corresponding orders k1 i being −3rd order, −2nd order,−1st order, zero-order, 1st order, 2nd order, and 3rd order. Note thatFIG. 43 illustrates calculation results assuming that the coefficient C₁is 1×10⁻² which is the maximum, and f_(all1) representing the overallfocal distance f_(all) calculated by Expression (4) of the firstwavelength λ1 is f_(all1)=2.2 (mm). The diffraction order has thus beendescribed above, but actually, geometrical optics can be applied to theinner ring zone portion alone, and the properties such as the focaldistance and so forth are determined with the inner ring zone portion,so the above-mentioned k1 through k3 correspond to k1 i through k3 i,and in other words, the above-mentioned relation of k1 through k3 alsohas the relation where k1 through k3 are substituted for k1 i through k3i respectively. According to FIG. 43, in order to set f3 to 2.5 mm orlonger, the relation of the following Expression (5A) holds.Accordingly, in order to ensure appropriate focal distance and operatingdistance, it is necessary to have the relation of the followingExpression (5B) from the above-mentioned relation of Expression (2B).k1i>k3i  (5A)k1i≧k2i>k3i  (5B)

Further, from an perspective wherein this Expression (5B) and alater-described restriction that a diffraction order to be employed isequal to or smaller than around 3, each of combinations of (k1 i, k3i)=(−2, −3), (−1, −2), (−1, −3), (0, −2), (0, −3), (1, −2), (1, −3), (2,−1), (2, −2), (2, −3), (3, 0), (3, −1), (3, −2), and (3, −3) is asuitable combination from the above-mentioned perspective. At this time,k2 i determined so as to satisfy Expression (5B) is employed. Note that,strictly, the relation in FIG. 43 is changed with the value of f1 andmaterial dispersion, and further, the target value of f3 deteriorates bydeteriorating f1, or changing incident magnification to the object lensto a divergent ray, but the above-mentioned choices of diffractionorders are suitable.

Next, description will be made regarding the third perspective. As thethird perspective, the configuration needs to be advantageous inmanufacturing. In a case wherein a diffraction order to be selected istoo great, the steps of the diffraction structure to be formed, and thedepth of blaze become deep. Further, when the depth of the diffractionstructure becomes deep, there is a possibility that formation precisiondeteriorates, and also there is a possibility that a problem occurswherein an optical path length enhancement effect due to change intemperature increases, and temperature diffraction efficiency propertiesdeteriorate. Also, there is a problem wherein deterioration in formationprecision leads to deterioration in diffraction efficiency. It isdesirable and common from such reasons to select a diffraction order upto around 3rd through 4th. Accordingly, with the above-mentioned secondperspective, study has been made employing a diffraction order up to3rd.

Next, description will be made regarding the fourth perspective. As thefourth perspective, though similar to the third perspective, thediffraction structure needs to be able to be manufactured. Whenperforming a diffraction efficiency calculation described in alater-described section of “Depth and shape of diffraction structure anddiffraction efficiency”, the depth d needs to be equal to or smallerthan a suitable size, and the diffraction structure needs to be formedwith this depth. Further, the depth d needs to be equal to or smallerthan at least 15 μm.

From the above-mentioned first through fourth perspectives, the firstdiffraction region 251 which is an inner ring zone is configured so asto generate each diffracted light having relation of k1 i≧k2 i>k3 i.

Further, the first diffraction region 251 is configured such that, ofthe diffraction orders k1 i, k2 i, and k3 i of each wavelength of whichthe diffraction efficiency is the maximum, k1 i and k3 i have any of thefollowing relations.

(k1 i, k3 i)=(−2, −3), (−1, −2), (−1, −3), (0, −2), (0, −3), (1, −2),(1, −3), (2, −1), (2, −2), (2, −3), (3, 0), (3, −1), (3, −2), and (3,−3)

Also, from the first through fourth perspectives, specifically, asdescribed later, the optimal configuration example is a case wherein (k1i, k2 i, k3 i)=(1, −1, −2), (0, −1, −2), (1, −2, −3) or (0, −2, −3).Now, when the diffraction orders k1 i, k2 i, and k3 i are selected asabove, the number of steps S and groove depth d selected at the time ofdiffraction efficiency and so forth being taken into consideration areshown in I1 through I4 in Table 6. Also, in Table 6, additionally, withthe relation of the plots Pλ1, Pλ2, and Pλ3, and design line L describedwith reference to FIG. 41, a later-described deviation amount Δ from thedesign line L of the plot Pλ3 indicating the third wavelength is shownin Table 6. That is to say, as shown in later-described FIG. 48, whensetting a line connecting the plots Pλ1 and Pλ2 (hereafter, referred toas “spherical aberration correction line”), this deviation amount Δindicates the distance deviated in the vertical axis direction(direction indicating protective layer thickness) from the plot Pλ3toward the spherical aberration correction line thereof. Here, in thecase of the deviation amount Δ=0, this indicates that the respectivepoints Pλ1, Pλ2, and Pλ3 are on a straight line completely. Also, in thecase of the deviation amount Δ is positive, this indicates that the plotPλ3 is positioned lower than the spherical aberration correction line,and in the case of the deviation amount A is negative, this indicatesthat the plot Pλ3 is positioned upper than the spherical aberrationcorrection line. Note here that in FIG. 41 illustrating the firstembodiment of an inner ring zone, it is difficult to illustrate thisdeviation amount Δ from the features of inner ring zones, so descriptionhas been made regarding this deviation amount Δ using FIG. 48 employedfor the first embodiment of an middle ring zone, but let us say that thedefinition regarding this deviation amount Δ is true for both inner ringzones and middle ring zones. As shown in Table 6, in any example,diffraction efficiency is sufficiently ensured, and the deviation amountΔ is also sufficiently small, and accordingly, a suitable diffractionorder can be confirmed even if spherical aberration correction is takeninto consideration.

TABLE 6 Order, diffraction efficiency, diffraction order, depth, numberof steps, deviation amount Δ of inner ring zones No. k_(1i) k_(2i)k_(3i) eff₁ eff₂ eff₃ d [μm] s Δ [mm] I1 1 −1 −2 0.81 0.62 0.57 3.8 4−0.06 I2 0 −1 −2 0.98 0.78 0.39 6.9 3 0.21 I3 1 −2 −3 0.86 0.70 0.52 5.46 −0.19 I4 0 −2 −3 0.86 0.50 0.39 4.0 5 −0.10

Next, description will be made regarding “Calculation of depth and shapeof diffraction structure and diffraction efficiency” with the firstdiffraction region 251 and so forth with reference to a specificembodiment. Now, a diffraction face design example such that thediffracted light of each order described above is taken as the maximumdiffracted light will be shown as the inner ring zone according to thefirst embodiment with reference to FIG. 44. Note that the diffractionamount (diffraction efficiency) of the selected diffraction orderfluctuates depending on groove depth such as shown in FIG. 44, sosetting suitable groove depth enables the diffraction efficiency of theselected diffraction order at each wavelength to be increased up to adesired level.

Specifically, FIGS. 44A through 44C illustrate change in diffractionefficiency as to the groove depth d when assuming that the diffractionstructure is the staircase form of the number of steps S=4, and (k1 i,k2 i, k3 i)=(+1, −1, −2). FIG. 44A is a diagram illustrating change indiffraction efficiency of +1st order diffracted light of the opticalbeam of the first wavelength, FIG. 44B is a diagram illustrating changein diffraction efficiency of −1st order diffracted light of the opticalbeam of the second wavelength, and is also a diagram illustrating changein diffraction efficiency of −2nd order diffracted light serving asunwanted light as described later, and FIG. 44C is a diagramillustrating change in diffraction efficiency of −2nd order diffractedlight of the optical beam of the third wavelength, and is also a diagramillustrating change in diffraction efficiency of +3rd order diffractedlight serving as unwanted light as described later. In FIGS. 44A through44C, the horizontal axis represents groove depth (nm), and the verticalaxis represents diffraction efficiency (light intensity). If we say thatthe diffraction efficiency of k1 i is eff1, the diffraction efficiencyof k2 i is eff2, and the diffraction efficiency of k3 i is eff3, theposition of the groove depth d=3800 (nm) shown in the horizontal axishas sufficient diffraction efficiency. Specifically, as shown in FIG.44A eff1=0.81, as shown in FIG. 44B eff2=0.62, and as shown in FIG. 44Ceff3=0.57, which have sufficient diffraction efficiency. As shown inFIGS. 44A through 44C, the relation between diffraction efficiency andgroove depth fluctuates depending on the number of steps, so there is aneed to select a suitable number of steps, but the number of steps S=4is employed here, as described above.

With the first diffraction region 251, the inner ring zone region isconfigured of a step structure (diffraction structure of staircaseform), which is a configuration suitable for deviating the diffractionefficiency of unwanted light generated at this diffraction region fromthe diffraction efficiency eff1, eff2, and eff3 of regular light. Now,let us say that the term “regular light” means diffracted light of thediffraction orders k1 i, k2 i, and k3 i thus selected, i.e., thediffracted light of a diffraction order of which the diffractionefficiency becomes the maximum, and the term “unwanted light” means thediffracted light of a diffraction order of which the diffractionefficiency becomes the second largest diffraction efficiency. Note thatin FIGS. 44A through 44C, and later-described FIGS. 45A through 45C and54A through 54C, LM represents change in the diffraction efficiency ofthe diffracted light of the diffraction order of which the diffractionefficiency becomes the maximum, and LF represents change in thediffraction efficiency of the diffracted light of the diffraction orderserving as unwanted light described here.

Description will be made wherein with the first diffraction region 251,the diffraction structure having the staircase form is formed, wherebythe influence of unwanted light can be reduced. In order to compare tothis FIGS. 44A through 44C, the diffraction efficiency in the case ofthis inner ring zone being formed as a blazed shape is illustrated inFIGS. 45A through 45C as a reference example. FIGS. 45A through 45Cillustrate change in the diffraction efficiency as to the groove depth dwhen assuming that the diffraction structure is formed as a blazed shapeof the number of steps S=∞, and (k1 i, k2 i, k3 i)=(+1, +1, +1). FIG.45A is a diagram illustrating change in the diffraction efficiency ofthe +1st order diffracted light of the optical beam of the firstwavelength, FIG. 45B is a diagram illustrating change in the diffractionefficiency of the +1st order diffracted light of the optical beam of thesecond wavelength, and also illustrating change in the diffractionefficiency of the zero-order light serving as unwanted light, and FIG.45C is a diagram illustrating change in the diffraction efficiency ofthe +1st order diffracted light of the optical beam of the thirdwavelength, and also illustrating change in the diffraction efficiencyof the zero-order light serving as unwanted light. In FIG. 45A through45C, the horizontal axis represents groove depth (nm), and the verticalaxis represents diffraction efficiency (light intensity). As shown inFIGS. 45A through 45C, in the case of the second and third wavelengths,the zero-order light has efficiency as unwanted light. With each opticalbeam of adjacent diffraction orders such as the zero-order light and 1storder light, diffraction angles have few differences. Thus, when theregular light which is either optical beam of the selected diffractionorders k2 i and k3 i is condensed on the corresponding optical disc soas to be in a focused state, unwanted light is also condensed in ablurring state. Subsequently, this unwanted light is also reflected atthe optical disc, and the reflection light of the unwanted light isirradiated on a photoreceptor portion, which has adverse influence upona signal obtained at the photoreceptor portion, and there is apossibility that jittering or the like will deteriorate. Further, thereis a possibility that this unwanted light leads to a problem wherein inthe case of defocus occurring, the influence thereof becomes great. Asshown in FIGS. 44A through 44C described above, the diffractionstructure having the staircase form is formed, whereby the diffractionefficiency of unwanted light can be reduced as compared to the caseshown in FIGS. 45A through 45C.

That is to say, in a case wherein an inner ring zone portion such as thefirst diffraction region 251 is formed in a staircase form, a structurecan be realized whereby the quantity of the diffracted light of unwantedlight is suppressed. With the diffraction structure having the staircaseform, groove depth which deteriorates the efficiency of unwanted lightcan be selected, and even if unwanted light efficiency becomes highefficiency, the order serving as regular light and the order serving asunwanted light differ greatly, whereby unwanted light can be preventedfrom condensing at the time of focus. Specifically, as shown in FIG.44B, the unwanted light efficiency according to the second wavelengthcan be suppressed to around 5% which does not contribute. Also, as shownin FIG. 44C, the regular light according to the third wavelength is −2ndorder light, but unwanted light is +3rd order light, and with this −2ndorder light and +3rd order light, diffraction angles differs greatly, sounwanted light is defocused greatly in the case of regular light beingfocused, and accordingly, there is no bad influence due to unwantedlight being input to the photoreceptor portion. In other words, aso-called step structure such as a staircase form is a structuresuitable for deviating the diffraction efficiency of regular light fromthe diffraction efficiency of the diffracted light of the adjacentorders as compared to a blazed form or the like.

Next, description will be made regarding “pitch design” according to thefirst diffraction region 251 and so forth. With the pitch design of thediffraction structure, if we say that a phase to be provided by adiffraction unit (diffraction face) having a predetermined diffractionstructure is φ, the phase thereof can be represented as the followingExpression (6) using the phase difference function coefficient Cn. Notethat in Expression (6), k represents the diffraction order to beselected at the respective wavelengths λ1, λ2, and λ3, and specifically,represents k1, k2, and k3, r represents a position in the radialdirection, and λ0 represents a design wavelength. Now, let us say thatin the case of λ0 employed for pitch design, calculation is performedassuming k=1.

$\begin{matrix}{\phi = {k{\sum\limits_{i = 0}^{n}\frac{C_{n}r^{2i}}{\lambda_{0}}}}} & (6)\end{matrix}$

In Expression (6), the value of φ can be obtained uniquely at the timeof lens design. On the other hand, φ represents the phase of the designwavelength λ0, and with φ′ obtained from the relational expression ofφ′=φ−nλ0, and the phase obtained by this φ, the given influence thereofis completely the same. In other words, φ′ obtained from theabove-mentioned relational expression is, as shown in FIG. 46B, is aremainder in the case of dividing φ by λ0 such as shown in FIG. 46A forexample, i.e., a value obtained by a so-called remainder calculation.This φ′ can be referred to as phase amount to be provided fordetermining the pitches of the actual diffraction structure. The actualdiffraction structure pitches are determined from this φ′, andspecifically, as shown in FIG. 46C, are determined so as to be alongwith the shape of this φ′. Note that the horizontal axes in FIG. 46Athrough 46C represent a position in the radial direction, the verticalaxis in FIG. 46A represents necessary phase amount φ for each positionthereof, the vertical axis in FIG. 46B represents granting phase amountφ′ obtained by remainder calculation for each position thereof, and thevertical axis in FIG. 46C represents groove depth d. Here, in FIG. 46C,after pitches are determined, a blazed shape is illustrated, but in thecase of employing a staircase form such as the above-mentioned firstdiffraction region 251 or the like, the blazed slope portion shown inFIG. 46C is formed in a staircase form of a predetermined number ofsteps S.

Note that description has been made above assuming that of thediffraction structure provided in the first diffraction region 251, thecross-sectional shape including the radial direction and optical axisdirection thereof has, as shown in FIG. 40A, the diffraction structureof multiple staircase forms formed with predetermined height andpredetermined width set generally with an equal interval within onestaircase portion, but the present invention is not restricted to this,an non-cyclical step form may be formed such that the height and/orwidth of a staircase form serving as reference is finely adjusted basedon an acquisition target phase such as shown in FIG. 46B. Further, aform determined by phase design may be formed so as to providepredetermined phase difference to the optical beam of a predeterminedwavelength, i.e., the cross-sectional shape may not be formed of only astraight line parallel to a horizontal line indicating a plane servingas reference, and a perpendicular line, but may be formed so as to be annon-cyclical form including a straight line (sloping surface) inclinedas to that straight line, curve (curved surface), or the like. Thispoint is true for a later-described second diffraction region 252.

A second diffraction region 252 which is a middle ring zone, where asecond diffraction structure is formed, which is a structure differentfrom the first diffraction structure having a ring zone shape andpredetermined depth, diffracts the optical beam of the first wavelengththat is transmitted therethrough such that diffracted light of an orderwhich forms an appropriate spot on the signal recording face of thefirst optical disc via the object lens 234 is dominant, i.e., such thatmaximum diffraction efficiency is manifested regarding diffracted lightof other orders.

Also, the second diffraction region 252 diffracts the optical beam ofthe second wavelength that is transmitted therethrough such thatdiffracted light of an order which forms an appropriate spot on thesignal recording face of the second optical disc via the object lens 234is dominant, i.e., such that maximum diffraction efficiency ismanifested regarding diffracted light of other orders, by way of thesecond diffraction structure.

Also, the second diffraction region 252 diffracts the optical beam ofthe third wavelength that is transmitted therethrough such thatdiffracted light of orders other than an order which forms anappropriate spot on the signal recording face of the third optical discvia the object lens 234 is dominant, i.e., such that maximum diffractionefficiency is manifested regarding diffracted light of other orders, byway of the second diffraction structure. If it puts in another wayregarding this point, in light of later-described flaring operation andso forth, the second diffraction region 252 diffracts the optical beamof the third wavelength that is transmitted therethrough such thatdiffracted light of an order which forms no appropriate spot on thesignal recording face of the third optical disc via the object lens 234is dominant, by way of the second diffraction structure. Note that thesecond diffraction region 252 can sufficiently reduce diffractionefficiency diffracted light of an order which forms an appropriate spoton the signal recording face of the third optical disc via the objectlens 234 for the optical beam of the third wavelength that istransmitted therethrough, by way of the second diffraction structure.

Thus, with the second diffraction region 252, there is formed adiffraction structure suitable for the diffracted light of apredetermined order being dominant as to the optical beam of theabove-mentioned respective wavelengths, thereby enabling sphericalaberration to be corrected and reduced at the time of the optical beamsof the first and second wavelengths serving as the diffracted light of apredetermined order that is transmitted therethrough being condensed onthe signal recording face of the corresponding optical disc via theobject lens 234.

Also, the second diffraction region 252 is configured so as to functionas described above as to the optical beams of the first and secondwavelengths, and is configured such that the diffracted light of anorder that does not condense the optical beam of the third wavelengththat is transmitted therethrough upon the signal recording face of thethird optical disc via the object lens 234 is dominant by taking intoconsideration the influence of flaring and so forth, so even if theoptical beam of the third wavelength that has transmitted the seconddiffraction region 252 is input to the object lens 234, this seldomaffects the signal recording face of the third optical disc, i.e., thesecond diffraction region 252 can serve so as to markedly reduce thelight quantity of the optical beam of the third wavelength transmittedthrough the second diffraction region 252, and condensed on the signalrecording face by the object lens 234 to around zero, and subject theoptical beam of the third wavelength to aperture restriction.

Incidentally, the above-mentioned first diffraction region 251 is formedwith a size wherein the optical beam of the third wavelength transmittedthrough the region thereof is input to the object lens 234 in the samestate as that of the optical beam subjected to aperture restriction ataround NA=0.45, and also, the second diffraction region 252 formed onthe outer side of the first diffraction region 251 does not condense theoptical beam of the third wavelength transmitted through this region onthe third optical disc via the object lens 234, so consequently, thediffraction unit 250 including the first and second diffraction regions251 and 252 thus configured serves so as to perform aperture restrictionat around NA=0.45 as to the optical beam of the third wavelength. Anarrangement has been made here wherein with the diffraction unit 250,aperture restriction of numerical aperture NA of around 0.45 isperformed as to the optical beam of the third wavelength, but thenumerical aperture restricted by the above arrangement is not restrictedto this.

Specifically, the second diffraction region 252 has, as shown in FIG. 39and FIG. 40B, a ring zone shape centered on the optical axis, which isformed such that the cross-sectional shape of this ring zone becomes ablazed shape of a predetermined depth (hereafter, also referred to as“groove depth”) d as to the reference face.

Also, description will be made here assuming that the second diffractionregion having a diffraction structure is formed such that thecross-sectional shape of the ring zone is a blazed shape, but as long asthis diffraction structure is configured such that the optical beam of apredetermined order is dominant as to the optical beam of eachwavelength as described above, for example, a diffraction region 252Bmay be formed, as shown in FIG. 47, which has a ring zone shape centeredon the optical axis, and the cross-sectional shape of this ring zone isconfigured as to the reference face such that staircase forms having apredetermined depth d, and a predetermined number of steps S are formedconsecutively in the radial direction.

As shown in FIG. 47, the diffraction region 252B in the case of astaircase form being formed as a middle ring zone has a ring zone shapecentered on the optical axis, and the cross-sectional shape of this ringzone is configured wherein staircase forms having a predetermine depth dand a predetermined number of steps S are consecutively formed in theradial direction. Note that the second diffraction region 252B hasdifferent numeric values of d and/or S as compared with those in thecase of the first diffraction region 251, i.e., the second diffractionstructure different from the first diffraction structure provided in thefirst diffraction region 251 is formed. For example, the diffractionstructure of the second diffraction region 252B shown in FIG. 47 is adiffraction structure wherein the number of steps S is set to 5 (S=5),staircase forms including first through fifth step portions 252 Bs 1,252 Bs 2, 252 Bs 3, 252 Bs 4, and 252 Bs 5 each having generally thesame depth (d/3) are consecutively formed in the radial direction, andfirst through sixth diffractive faces 252 Bf 1, 252 Bf 2, 252 Bf 3, 252Bf 4, 252 Bf 5, and 252 Bf 6 are formed generally with the same interval(d/5) in the optical axis direction.

Also, in a case wherein the second diffraction region 252 diffracts theoptical beam of the first wavelength which is transmitted therethroughsuch that diffracted light of the k1 m'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, diffracts theoptical beam of the second wavelength which is transmitted therethroughsuch that diffracted light of the k2 m'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, and diffracts theoptical beam of the third wavelength which is transmitted therethroughsuch that diffracted light of the k3 m'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, the diffractionorders k1 m, k2 m, and k3 m are set so as to satisfy relationsdetermined from the following first through third perspectives.

First, the first perspective will be described. As the firstperspective, the diffraction orders k1 m, k2 m, and k3 m which becomethe maximum diffraction efficiency do not satisfy the relationalexpression of the above-mentioned Expression (1) (let us say that x ofk1 x, k2 x, k3 x within this conditional expression with a middle ringzone is x=m). This is because with a middle ring zone region, in thecase of k1 m, k2 m, and k3 m satisfying Expression (1), the diffractedlight of the order k3 m of the third wavelength is formed on the signalrecording face of the third optical disc. In such a case, the aperturerestriction as to the third wavelength cannot be realized.

In other words, an arrangement may be made wherein the seconddiffraction region 252 generates the diffraction efficiency of thediffracted light of the diffraction orders k1 m and k2 m of the opticalbeams of the first and second wavelengths in a high state via the objectlens 234 so as to condense light to form a suitable spot on the signalrecording faces of the first and second optical discs, and suppressesthe diffraction efficiency of the diffraction order of the optical beamof the third wavelength condensed on the signal recording face of thethird optical disc as much as possible so as to have an aperturerestriction function, but the relation of Expression (1) is notsatisfied here, thereby shifting the optical beam of the diffractionorder according to the optical beam of this third wavelength from astate where a focal point is imaged on the signal recording face of thethird optical disc to further reduce the light quantity of the opticalbeam condensed on the signal recording face of the third optical discsubstantially. Hereafter, a position where the optical beam of apredetermined wavelength is formed via the object lens 234 is shiftedfrom the signal recording face of the corresponding optical disc,thereby reducing the light quantity of the optical beam of thiswavelength condensed on the signal recording face substantially, whichwill be called flaring, and the details thereof will be described later.

Note that with regard to the third wavelength, there is a need to makean arrangement such that with not only the diffraction order k3 m havingthe maximum diffraction efficiency but also all of the diffractionorders having predetermined diffraction efficiency, the diffractionorders thereof will be replaced with k3 m, and the above-mentionedrelational expression is set so as not to be satisfied along with k1 m,and k2 m. This is because if the diffracted light of the diffractionorder having predetermined efficiency satisfies the relation ofExpression (1), the diffracted light thereof is condensed by the objectlens, and accordingly aperture restriction cannot be performedappropriately. Now, let us say that the term “predetermined diffractionefficiency” means an efficiency level wherein the optical beamtransmitted through this region is irradiated on the optical disc, theoptical beam reflected at the optical disc is input to the photoreceptorportion, and this becomes noise when the return light of the opticalbeam transmitted within a regular aperture range is detected at thephotoreceptor unit, and in other words, means an efficiency levelwherein aperture restriction cannot be performed appropriately.

On the other hand thereof, like this first perspective, the diffractionorders k1 m, k2 m, and k3 m that do not satisfy the relationalexpression of Expression (1) are selected, whereby aperture restrictionas to the third wavelength can be performed appropriately.

Next, the second perspective will be described. As the secondperspective, in a case wherein similar to the description regardinginner ring zones, the selected order is too great, the steps, groovewidth, and blazed depth of the diffraction structure becomes deeper.When the depth of the diffraction structure becomes deeper, there is apossibility that formation precision deteriorates, and also there isproblem wherein the optical path length enhancement increase effectaccording to change in temperature increases, temperature diffractionefficiency properties deteriorate. It is desirable and common from suchreasons to select an diffraction order up to around 3rd to 4th order.

Next, the third perspective will be described. As the third perspective,similar to the description regarding inner ring zones, when diffractionefficiency calculation as described later is performed, there is a needto satisfy that the depth d is equal to or smaller than a suitable size,and formation can be made with this depth size. Further, the depth dneeds to be equal to or smaller than at least 15 μm.

Predetermined diffraction orders k1 m and k2 m need to be selected so asto satisfy the above-mentioned first through third perspectives at thesecond diffraction region, and for example, a combination of (k1 m, k2m)=(+1, +1), (−1, −1), (0, +2), (0, −2), (0, +1), (0, −1), (+1, 0), and(−1, 0) (hereafter, this combination is referred to as “diffractionorder combination A of middle ring zones”), and a combination of (k1 m,k2 m)=(+3, +2), (−3, −2), (+2, +1), and (−2, −1) (hereafter, thiscombination is referred to as “diffraction order combination B of middlering zones”) are optimal arrangement examples. Now, the following Table7 shows the above-mentioned functions of middle ring zones, staircaseforms when taking into consideration diffraction efficiency and soforth, diffraction structure form selected from blazed forms, number ofsteps S (“∞” in the case of a blazed form), and groove depth d, whenselecting the diffraction order combination A or B of middle ring zones.As shown in Table 7, with the diffraction order combination A of middlering zones, there is groove depth whereby the optimal diffractionefficiency can be obtained with the diffraction structure of thestaircase form which is a so-called step form, i.e., it can be said thatthis combination is a combination suitable for the diffraction structureof the staircase form. In Table 7, MA1 through MA4 show respectivecombinations of the combination A, and MB1 through MB2 show respectivecombinations of the combination B. Note that in the case of thecombination A, the optimal solution can be obtained even with annon-cyclical structure. Also, with the diffraction order combination Bof middle ring zones, there is groove depth whereby the optimaldiffraction efficiency can be obtained with the diffraction structure ofthe blazed form, i.e., it can be said that this combination is acombination suitable for the diffraction structure of the blazed form.Note that in Table 7, with the diffraction structure suitable for theabove-mentioned combination of the diffraction orders k1 m and k2 m, adiffraction order k3 m of which the diffraction efficiency of theoptical beam of the third wavelength becomes the maximum efficiency, anda diffraction order having the second largest diffraction efficiency asso-called unwanted light is shown as “k3 m′”. Also, in Table 7,diffraction efficiency eff1, eff2, and eff3 of the orders k1 m, k2 m,and k3 m of the respective wavelengths, and also diffraction efficiencyeff3′ of the diffraction order k3 m′ of the third wavelength are shown.Further, in a case wherein with each example, the deviation amount Δfrom the spherical aberration correction line of the plot Pλ3 of thethird wavelength, and also in the case of plotting the diffraction orderk3 m′ of the third wavelength similarly, the deviation amount from thespherical aberration correction line of this plot point is shown as“Δ′”. Note that the combinations of Table 7 and the orders k1 m, k2 m,k3 m, and k3 m′ within later-described Table 8 are combinations ofdecoding in the same order. Also, in Table 7, the asterisk “*” indicatesthat with eff3′, diffraction efficiency is low, which effects noproblem.

TABLE 7 Order, diffraction efficiency, diffraction order, depth, numberof steps, deviation amount Δ of middle ring zones No. K_(1m) k_(2m)k_(3m) k_(3m)′ eff₁ eff₂ eff₃ eff₃′ d [μm] s Δ [mm] Δ′ [mm] MA1 ∓1 ∓1 0※ 0.80 0.48 0.52 ※ 6.4 3 −1.83 ※ MA2 0 ∓2 0 ∓2 1.00 0.57 0.25 0.23 3.1 4−1.01 −0.40 MA3 0 ∓1 ±1 ∓1 0.99 0.63 0.28 0.28 1.6 2 −1.62 −0.40 MA4 ∓10 0 ∓1 0.79 0.85 0.43 0.34 4.1 3 −0.50 −1.49 MB1 ±3 ±2 ±2 ±1 0.96 0.930.47 0.34 2.4 ∞ 0.75 −3.15 MB2 ±2 ±1 ±1 ※ 1.00 0.86 1.00 ※ 1.6 ∞ −0.93 ※

As shown in Table 7, with the above-mentioned combinations A and B, ineither case, diffraction efficiency is sufficiently ensured, and also inthe case of the diffraction efficiency of the third wavelength existing,the deviation amount Δ is sufficiently great, i.e., spherical aberrationis provided greatly to the optical beam of the third wavelength, whichdoes not contribute to image formation, thereby confirming that theaperture restriction function is exhibited. This means that flaringeffects are obtained. Note that in Table 7, with the combinations A andB, it goes without saying that there is a combination including multiplesolutions as to the groove depth d and number of steps S, but an exampleof the groove depth d and number of steps S thereof is shown as atypical example thereof.

Also, the diffraction orders k1 m and k2 m to be selected at the seconddiffraction region 252 that satisfy the above-mentioned first throughthird perspectives are not restricted to the above combinations, and forexample, a combination of (k1 m, k2 m)=(+1, −1) and (−1, +1) (hereafter,this combination is referred to as “diffraction order combination C ofmiddle ring zones”), and a combination of (k1 m, k2 m)=(+1, +1) and (−1,−1) (hereafter, this combination is referred to as “diffraction ordercombination D of middle ring zones”) are also optimal arrangementexamples. Now, when selecting the diffraction order combination C or Dof the middle ring zones, the above-mentioned functions of the middlering zones, staircase form to be selected when taking into considerationdiffraction efficiency and so forth, the form of the diffractionstructure to be selected from blazed forms, number of steps S, andgroove width d are shown in MC1 and MD1 in the following Table 8. Now,as shown in Table 8, with the diffraction order combination C of themiddle ring zones, there is groove depth whereby the optimal diffractionefficiency can be obtained with the diffraction structure of thestaircase form which is a so-called step form, i.e., it can be said thatthis combination is a combination suitable for the diffraction structureof the staircase form. Also, with the diffraction order combination D ofmiddle ring zones, there is groove depth whereby the optimal diffractionefficiency can be obtained with the diffraction structure of the blazedform, i.e., it can be said that this combination is a combinationsuitable for the diffraction structure of the blazed form. Note that “k1m”, “k2 m”, “k3 m”, “k3 m”, “eff1”, “eff2”, “eff3”, “eff3′”, “d”, “S”,“Δ”, and “Δ′” shown in Table 8 are the same as those described abovewith reference to Table 7.

TABLE 8 Order, diffraction efficiency, diffraction order, depth, numberof steps, deviation amount Δ of middle ring zones No. K_(1m) k_(2m)k_(3m) k_(3m)′ eff₁ eff₂ eff₃ eff₃′ d [μm] s Δ [mm] Δ′ [mm] MC1 ±1 ∓1 ∓1∓2 0.81 0.81 0.32 0.19 2.9 3 −0.44 −0.06 MD1 ±1 ±1 ±1 0 1.00 0.60 0.420.39 0.8 ∞ −0.25 −1.83

As shown in Table 8, with the above-mentioned combinations C and D, ineither case, diffraction efficiency is sufficiently ensured. Note thatwith the example shown in Table 8, the deviation amount Δ or Δ′ is notsufficiently great amount as compared to the example shown in Table 7,but comparatively low diffraction efficiency eff3 and eff3′, and acertain level of separation amount Δ and Δ′ are obtained, so influenceof unwanted light can sufficiently be reduced while realizing aperturerestriction, for example, using a method for setting returnmagnification of an optical system greatly, or the like.

As described above, with the second diffraction region 252 serving as aninner ring zone, from the above-mentioned first through fourthperspectives, the diffraction order combination A, B, C, or D of innerring zones such as describe above can be selected, and such adiffraction order is selected, whereby the optical beams of the firstand second wavelengths can be condensed on the signal recording face ofthe corresponding optical disc with high diffraction efficiency in astate in which spherical aberration is reduced, and also the diffractedlight of the high diffraction order of diffraction efficiency isprevented from being condensed on the signal recording face of the thirdoptical disc as to the optical beam of the third wavelength, therebyenabling aperture restriction to be performed.

Note that, as described above, with a middle ring zone, the seconddiffraction region 252B of the staircase form may be employed instead ofthe second diffraction region 252 of the blazed form. This is because,as described in the above description of inner ring zones, while thestaircase form (step structure) is advantageous to reduce influence ofunwanted light, middle ring zones are provided outer side than innerring zones, and the lens curved face is steep, so the blazed form(blazed structure) is advantageous from the perspective ofmanufacturing. That is to say, with a middle ring zone, an advantageousstructure needs to be selected while taking into consideration therelation with other structures with subtle balance between influence ofunwanted light and advantages from the perspective of manufacturing.

Now, description will be made regarding flaring with the seconddiffraction region 252, and the structure thereof. With the abovedescription of the first diffraction region 251, description has beenmade wherein it is required to satisfy the above-mentioned conditionalexpression (λ1×k1 x−λ2×k2 x)/(t1−t2)≈(λ1×k1 x−λ3×k3 x)/(t1−t3), but thisconditional expression (with a middle ring zone, let us say that x of k1x, k2 x, and k3 x within this conditional expression is x=m) is alsotaken into consideration with the second diffraction region 252. Withthe second diffraction region 252 serving as a middle ring zone, whentaking into consideration a function for generating the diffractionlight of the diffraction orders k1 m and k2 m of the optical beams ofthe first and second wavelengths to be condensed via the object lens 234in a state wherein diffraction efficiency is high so as to form asuitable spot on the signal recording faces of the first and secondoptical discs such as described above, Pλ1 and Pλ2 to be plotted need tobe positioned on a design line, but further, in order to perform flaringregarding the third wavelength, there is a need to select a design lineso as to make Pλ3 deviate from this design line intentionally. That isto say, the object lens 234 is configured based on the design line thatdeviates from the design line regarding Pλ3, whereby the diffractedlight of the relevant diffraction order of the optical beam of the thirdwavelength can be shifted from a state wherein a focal point is imagedon the signal recording face of the third optical disc, the lightquantity of the optical beam of the third wavelength condensed on thesignal recording face of the third optical disc can be reducedsubstantially, whereby aperture restriction as to the optical beam ofthe third wavelength as described above can be performed in a sure andexcellent manner. Specifically, Pλ3 deviates from the design line L22 inthe case of (k1 m, k2 m, k3 m)=(+3, +2, +2) such as shown in FIG. 48,and in addition to the effects wherein the diffraction efficiency of thediffracted light of the relevant order of the third wavelength can bereduced according to the diffraction structure formed in the seconddiffraction region 252 expected from the beginning, the flaring effectsare further obtained, and according to such a configuration, the lightquantity of the optical beam of the third wavelength can be furtherprevented from being input to the third optical disc.

With the third diffraction region 253 which is an outer ring zone, thethird diffraction structure is formed, which has a ring zone shape,predetermined depth, and a structure different from the first and seconddiffraction structures, and the third diffraction region 253 diffractsthe optical beam of the first wavelength that is transmittedtherethrough such that diffracted light of an order which forms anappropriate spot on the signal recording face of the first optical discvia the object lens 234 is dominant, i.e., such that maximum diffractionefficiency is manifested regarding diffracted light of other orders.

Also, the third diffraction region 253 diffracts the optical beam of thesecond wavelength that is transmitted therethrough such that diffractedlight of an order other than the order which condenses light so as toform an appropriate spot on the signal recording face of the secondoptical disc via the object lens 234 is dominant, i.e., such thatmaximum diffraction efficiency is manifested regarding diffracted lightof other orders, by way of the third diffraction structure. If it putsin another way regarding this point, in light of later-described flaringoperation and so forth, the third diffraction region 253 diffracts theoptical beam of the second wavelength that is transmitted therethroughsuch that diffracted light of an order which forms no appropriate spoton the signal recording face of the second optical disc via the objectlens 234 is dominant, by way of the third diffraction structure. Notethat the third diffraction region 253 can sufficiently reduce thediffraction efficiency of diffracted light of an order which forms anappropriate spot condensed on the signal recording face of the secondoptical disc via the object lens 234 for the optical beam of the secondwavelength that is transmitted therethrough, by way of the thirddiffraction structure.

Also, the third diffraction region 253 diffracts the optical beam of thethird wavelength that is transmitted therethrough such that diffractedlight of an order other than which forms an appropriate spot condensedon the signal recording face of the third optical disc via the objectlens 234 is dominant, i.e., such that maximum diffraction efficiency ismanifested regarding diffracted light of other orders, by way of thethird diffraction structure. If it puts in another way regarding thispoint, in light of later-described flaring operation and so forth, thethird diffraction region 253 diffracts the optical beam of the thirdwavelength that is transmitted therethrough such that diffracted lightof an order which forms no appropriate spot on the signal recording faceof the third optical disc via the object lens 234 is dominant, by way ofthe third diffraction structure. Note that the third diffraction region253 can sufficiently reduce the diffraction efficiency of diffractedlight of an order which forms an appropriate spot condensed on thesignal recording face of the third optical disc via the object lens 234for the optical beam of the third wavelength that is transmittedtherethrough, by way of the third diffraction structure.

Thus, with the third diffraction region 253, there is formed adiffraction structure suitable for the diffracted light of apredetermined order being dominant as to the optical beam of theabove-mentioned respective wavelengths, thereby enabling sphericalaberration to be corrected and reduced at the time of the optical beamsof the first wavelength serving as the diffracted light of apredetermined order that is transmitted therethrough being condensed onthe signal recording face of the optical disc via the object lens 234.

Also, the third diffraction region 253 serves as described above as tothe optical beam of the first wavelength, and is configured such thatthe diffracted light of an order that does not condense the opticalbeams of the second and third wavelengths that is transmittedtherethrough upon the signal recording faces of the second and thirdoptical discs via the object lens 234 is dominant by taking intoconsideration the influence of flaring and so forth, so even if theoptical beams of the second and third wavelengths that have transmittedthe third diffraction region 253 is input to the object lens 234, thisseldom affects the signal recording faces of the second and thirdoptical discs, i.e., the third diffraction region 253 can serve so as tomarkedly reduce the light quantity of the optical beams of the secondand third wavelengths transmitted through the third diffraction region253, and condensed on the signal recording face by the object lens 234to around zero, and subject the optical beam of the second wavelength toaperture restriction. Note that the third diffraction region 253 canserve so as to subject the optical beam of the third wavelength toaperture restriction together with the above-mentioned seconddiffraction region 252.

Incidentally, the above-mentioned second diffraction region 252 isformed with a size wherein the optical beam of the second wavelengthtransmitted through the region thereof is input to the object lens 234in the same state as that of the optical beam subjected to aperturerestriction at around NA=0.6, and also, the third diffraction region 253formed on the outer side of the second diffraction region 252 does notcondense the optical beam of the second wavelength transmitted throughthis region on the optical disc via the object lens 234, soconsequently, the diffraction unit 250 including the second and thirddiffraction regions 252 and 253 thus configured serves so as to performaperture restriction at around NA=0.6 as to the optical beam of thesecond wavelength. An arrangement has been made here wherein with thediffraction unit 250, aperture restriction of numerical aperture NA ofaround 0.6 is performed as to the optical beam of the second wavelength,but the numerical aperture restricted by the above arrangement is notrestricted to this.

Also, the third diffraction region 253 is formed of a size such that theoptical beam of the first wavelength which has been transmitted throughthe region thereof is input to the object lens 234 in the same state asan optical beam which has been subjected to aperture restriction ataround NA=0.85, and since there is no diffraction structure formed onthe outer side of this third diffraction region 253, this does not allowcondensation of the optical beam of the first wavelength which has beentransmitted through this region on the first optical disc, and thediffraction unit 250 which has the third diffraction region 253configured thus functions so as to restrict the numerical aperture ofthe optical beam of the first wavelength to around NA=0.85. Note thatwith the first wavelength optical beam transmitted through the thirddiffraction region 253, light of 1st and 4th diffraction orders isdominant, so the zero-order light transmitted through the region outsidethe third diffraction region 253 almost never passes through the objectlens 234 to be condensed on the first optical disc, but in cases whereinthis zero-order does pass through the object lens 234 and is condensedon the first optical disc, a configuration may be provided to performaperture restriction by providing, at the region outside of the thirddiffraction region 253, either a shielding portion for shielding opticalbeams passing through, or a diffraction region having a diffractionstructure wherein optical beams of orders other than the order of theoptical beam passing through the object lens 234 to be condensed on thefirst optical disc are dominant. It should be noted however, that whilein this arrangement of the diffraction unit 250, the optical beam of thefirst wavelength is subjected to aperture restriction around NA=0.85,but the present invention is not restricted to this, i.e., numericalaperture restriction due to the above configuration is not limited tothis.

Specifically, the third diffraction region 253 has, as shown in FIG. 39and FIG. 40C, a ring zone shape centered on the optical axis, which isformed such that the cross-sectional shape of this ring zone becomes ablazed shape of a predetermined depth d as to the reference face.

With the third diffraction region 253 which is an outer ring zone, ablazed structure is employed, as described above. This is because withthe outer ring zone provided on the outermost side, the lens curved facehas the most steep curvature, and providing a structure other than ablazed structure is disadvantageous from the perspective ofmanufacturing. Also, there is no need to take into considerationproblems such as unwanted light, efficiency, and so forth as describedabove, so sufficient performance can be obtained with a blazedstructure. Description will be made below regarding the respectiveorders to be selected.

Also, in a case wherein the third diffraction region 253 diffracts theoptical beam of the first wavelength which is transmitted therethroughsuch that diffracted light of the k1 o'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, diffracts theoptical beam of the second wavelength which is transmitted therethroughsuch that diffracted light of the k2 o'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, and diffracts theoptical beam of the third wavelength which is transmitted therethroughsuch that diffracted light of the k3 o'th order is dominant, i.e., suchthat the diffraction efficiency thereof is maximum, when selecting thediffraction orders k1 o, k2 o, and k3 o, only the order of the firstwavelength and diffraction efficiency need to be taken intoconsideration.

This is because the condensed points of the second and third wavelengthshaving predetermined diffraction efficiency are subjected to flaring soas to be shifted from the state wherein an image is formed, whereby thelight quantity of the optical beams condensed on the signal recordingface of the second and third optical discs can be reduced substantially,and accordingly, flexibility is high, and conditions are alleviated.

From the perspectives described above, with the third diffraction region253, predetermined diffraction orders k1 o, k2 o, and k3 o need to beselected, as for an example thereof, like a later-described firstembodiment, in the case of (k1 o, k2 o, k3 o)=(+4, +2, +2), theabove-mentioned respective perspectives are satisfied, the correspondingefficiency can be obtained.

Now, description will be made regarding flaring with the thirddiffraction region 253, and the structure thereof. With the abovedescription of the first diffraction region 251, description has beenmade wherein it is required to satisfy the conditional expression (λ1×k1x−λ2×k2 x)/(t1−t2)≈(λ1×k1 x−λ3×k3 x)/(t1−t3), but this conditionalexpression (with an outer ring zone, let us say that x of k1 x, k2 x,and k3 x within this conditional expression is x=o) is also taken intoconsideration with the third diffraction region 253. With the thirddiffraction region 253 serving as an outer ring zone, when taking intoconsideration a function for generating the diffraction light of thediffraction order ko of the optical beams of the first wavelength to becondensed via the object lens 234 in a state wherein diffractionefficiency is high so as to form a suitable spot on the signal recordingface of the first optical disc such as described above, Pλ1 to beplotted needs to be positioned on a design line, but further, in orderto subject the second or third wavelength, or the second and thirdwavelengths to flaring, there is a need to select a design line so as tomake the corresponding Pλ2 and Pλ3 deviate from this design lineintentionally.

That is to say, the object lens 234 is configured based on the designline that deviates from the design line regarding Pλ2, whereby thediffracted light of the relevant diffraction order of the optical beamof the second wavelength can be shifted from a state wherein a focalpoint is imaged on the signal recording face of the second optical disc,the light quantity of the optical beam of the second wavelengthcondensed on the signal recording face of the second optical disc can bereduced substantially, whereby aperture restriction as to the opticalbeam of the second wavelength as described above can be performed in asure and excellent manner. Also, the object lens 234 is configured basedon the design line that deviates from the design line regarding Pλ3,whereby the diffracted light of the relevant diffraction order of theoptical beam of the third wavelength can be shifted from a state whereina focal point is imaged on the signal recording face of the thirdoptical disc, the light quantity of the optical beam of the thirdwavelength condensed on the signal recording face of the third opticaldisc can be reduced substantially, whereby aperture restriction as tothe optical beam of the third wavelength as described above can beperformed in a sure and excellent manner. Also, the object lens 234 isconfigured based on the design line that deviates from the design lineregarding Pλ2 and Pλ3, whereby both effects described above, i.e., thelight quantity of the optical beams of the second and third wavelengthscondensed on the signal recording face of the corresponding optical disccan be reduced.

Specifically, both of Pλ2 and Pλ3 deviate from the design line L23 inthe case of (k1 o, k2 o, k3 o)=(+4, +2, +2) such as shown in FIG. 49,and in addition to effects wherein the diffraction efficiency of thediffracted light of the orders of the second and third wavelengths canbe reduced according to the diffraction structure formed in the thirddiffraction region 253 expected from the beginning, the flaring effectsare further obtained, and according to such a configuration, the lightquantity of the optical beams of the second and third wavelengths can befurther prevented from being input to the second and third opticaldiscs.

As a specific embodiment of the diffraction unit 250 including the firstdiffraction region 251 which is an inner ring zone, the seconddiffraction region 252 which is a middle ring zone, and the thirddiffraction region 253 which is an outer ring zone, the diffractionorder of the diffracted light of an order that is dominant as to theoptical beam of each wavelength, and the diffraction efficiency of thediffracted light of the diffraction order thereof will be shown in Table9 and later-described Table 10 by listing specific numeric valuesregarding the depth d and number of steps S according to the blazed orstaircase form. Note that Table 9 shows the first embodiment of thediffraction unit 250, Table 10 shows the second embodiment of thediffraction unit 250, and in Tables 9 and 10, k1 represents thediffraction orders (k1 i, k1 m, k1 o) wherein the diffraction efficiencyof the optical beam of the first wavelength at each ring zone becomesthe maximum efficiency, i.e., the diffraction orders whereincondensation is made so as to form a spot appropriately on the signalrecording face of the first optical disc via the object lens 234, eff1represents the diffraction efficiency of the relevant diffraction orders(k1 i, k1 m, k1 o) of the optical beam of the first wavelength, k2represents the diffraction orders (k2 i, k2 m, k2 o) wherein thediffraction efficiency of the optical beam of the second wavelengthbecomes the maximum efficiency, and particularly with the inner andmiddle ring zones, represents the diffraction orders whereincondensation is made so as to form a spot appropriately on the signalrecording face of the second optical disc via the object lens 234, eff2represents the diffraction efficiency of the relevant diffraction orders(k2 i, k2 m, k2 o) of the optical beam of the second wavelength, k3represents the diffraction orders (k3 i, k3 m, k3 o) wherein thediffraction efficiency of the optical beam of the third wavelengthbecomes the maximum efficiency, and particularly with the inner ringzone, represents the diffraction orders wherein condensation is made soas to form a spot appropriately on the signal recording face of thethird optical disc via the object lens 234, eff3 represents thediffraction efficiency of the relevant diffraction orders (k3 i, k3 m,k3 o) of the optical beam of the third wavelength, d represents thegroove depth of each diffraction region, S represents the number ofsteps in the case of the staircase form, or “∞” in the case of theblazed form. Also, “*” in Tables 9 and 10 represents a state whereinaccording to the above-mentioned flaring, efficiency does not effect aproblem.

TABLE 9 Diffraction efficiency, diffraction order, depth, and number ofsteps at each ring zone, of First Embodiment k1 eff₁ K2 eff₂ K3 eff₃ d[μm] s Inner Ring zone 1 0.81 −1 0.62 −2 0.57 3.8 4 Middle Ring zone 30.96 2 0.93 2 ※ 2.4 ∞ Outer Ring zone 4 1.0 2 ※ 2 ※ 3.1 ∞ * representsthat according to flaring, efficiency does not effect a problem.

Now, the first embodiment shown in Table 9 will be described. With theinner ring zone according to the first embodiment, as shown in Table 9,when employing a staircase form with the number of steps S=4 and groovedepth d=3.8 (μm), with the diffraction order k1 i=+1 of the optical beamof the first wavelength, the diffraction efficiency is eff1=0.81, withthe diffraction order k2 i=−1 of the optical beam of the secondwavelength, the diffraction efficiency is eff2=0.62, and with thediffraction order k3 i=−2 of the optical beam of the third wavelength,the diffraction efficiency is eff3=0.57. Further specific description ofthe inner ring zone according to the first embodiment has been made withreference to FIGS. 44A through 44C, so detailed description thereof willbe omitted.

Also, with the middle ring zone according to the first embodiment, asshown in Table 9, when employing a blazed form (S=∞) with groove depthd=2.4 (μm), with the diffraction order k1 m=+3 of the optical beam ofthe first wavelength, the diffraction efficiency is eff1=0.96, and withthe diffraction order k2 m=+2 of the optical beam of the secondwavelength, the diffraction efficiency is eff2=0.93. Also, thediffraction efficiency eff3 of the diffraction order k3 m=+2 serving asthe maximum diffraction efficiency of the optical beam of the thirdwavelength transmitting this region is around 0.4, but this does notcontribute to image formation since the spot is subjected to flaring asdescribed above with reference to FIG. 48.

Next, description will be made further specifically regarding the middlering zone according to the first embodiment with reference to FIGS. 50Athrough 50C. FIG. 50A is a diagram illustrating change in thediffraction efficiency of +3rd order diffracted light of the opticalbeam of the first wavelength in the case of changing the groove depth dof the blazed form of the number of steps S=∞, FIG. 50B is a diagramillustrating change in the diffraction efficiency of +2nd orderdiffracted light of the optical beam of the second wavelength in thecase of changing the groove depth d of the blazed form of the number ofsteps S=∞, and FIG. 50C is a diagram illustrating change in thediffraction efficiency of +2nd order diffracted light of the opticalbeam of the third wavelength in the case of changing the groove depth dof the blazed form of the number of steps S=∞. In FIGS. 50A through 50C,the horizontal axis represents groove depth (nm), and the vertical axisrepresents diffraction efficiency (light intensity). At a position wherethe horizontal axis is 2400 nm, as shown in FIG. 50A, eff1 is 0.96, andas shown in FIG. 50B, eff2 is 0.93, and as shown in FIG. 50C, eff3 isaround 0.4, but the spot is subjected to flaring.

Also, with the middle ring zone in the first embodiment described above,of the design line in the relation between the above-described(wavelength×order) and the thickness of the protective layer, they-intercept position and inclination with the vertical axis representingthe thickness of the protective layer as the Y axis exhibits flaringregarding the third wavelength by change due to design of the objectlens. Accordingly, performing appropriate object lens design based onsuch a design line enables the quantity of light of the optical beam ofthe third wavelength to be further suppressed and excellent aperturerestriction to be performed regarding the optical beam of the thirdwavelength. Specifically, as shown in FIG. 48, the middle ring zone inthe first embodiment has the design line indicated by L22 set byplotting the points Pλ1, Pλ2, and Pλ3 at the diffraction orders (k1 m,k2 m, k3 m)=(+3, +2, +2). In FIG. 48 the design point Pλ1 of the firstwavelength and the design point Pλ2 of the second wavelength arepositioned on the design line L22, so the aberration of diffractionlight of the diffraction orders k1 m and k2 m is approximately zero. Onthe other hand, the plotted point Pλ3 of the third wavelength issignificantly deviated from the aberration zero design point, indicatingthe above-described flaring. Note that in FIG. 48, only k3 m=+2 is shownplotted, but there is deviation from the design line L22 in the same wayfor other orders in the third wavelength as well. Consequently, there isuncorrected aberration in the third wavelength, and consequently, thelight quantity of the optical beam of the third wavelength which haspassed through the middle ring zone, that is not imaged at the signalrecording face but input to the third optical disc can be suppressed. Asa result, regardless of the diffraction efficiency of the optical beamof the third wavelength as shown in FIG. 50, these optical beams do notcontribute to image formation, and accordingly, a suitable aperturerestriction (NA=0.45) can be realized.

Also, with the outer ring zone according to the first embodiment, asshown in Table 9, when employing a blazed form (S=∞) with groove depthd=3.1 (μm), with the diffraction order k1 o=+4 of the optical beam ofthe first wavelength, the diffraction efficiency is eff1=1.0. Also, thediffraction efficiency eff2 of the diffraction order k2 o=+2 serving asthe maximum diffraction efficiency of the optical beam of the secondwavelength transmitting this region is around 0.6, but this does notcontribute to image formation since the spot is subjected to flaring asdescribed above with reference to FIG. 49. Further, the diffractionefficiency eff3 of the diffraction order k3 o=+2 serving as the maximumdiffraction efficiency of the optical beam of the third wavelengthtransmitting this region is around 1.0, but this does not contribute toimage formation since the spot is subjected to flaring as describedabove with reference to FIG. 49.

Next, description will be made further specifically regarding the outerring zone according to the first embodiment with reference to FIGS. 51Athrough 51C. FIG. 51A is a diagram illustrating change in thediffraction efficiency of +4th order diffracted light of the opticalbeam of the first wavelength in the case of changing the groove depth dof the blazed form of the number of steps S=∞, FIG. 51B is a diagramillustrating change in the diffraction efficiency of +2nd orderdiffracted light of the optical beam of the second wavelength in thecase of changing the groove depth d of the blazed form of the number ofsteps S=∞, and FIG. 51C is a diagram illustrating change in thediffraction efficiency of +2nd order diffracted light of the opticalbeam of the third wavelength in the case of changing the groove depth dof the blazed form of the number of steps S=∞. In FIGS. 51A through 51C,the horizontal axis represents groove depth (nm), and the vertical axisrepresents diffraction efficiency (light intensity). At a position wherethe horizontal axis is 3100 nm, as shown in FIG. 51A, eff1 is 1.0, andas shown in FIG. 51B, eff2 is around 0.6, and as shown in FIG. 51C, eff3is around 1.0, but the spot is subjected to flaring.

Also, with the outer ring zone in the first embodiment described aboveas well, in the same way as the case of the middle ring zone in thefirst embodiment described above, an arrangement is made wherein thedesign line of the object lens is deviated, and flaring is carried outregarding the second and third wavelengths to perform excellent aperturerestriction. Specifically, as shown in FIG. 49, the outer ring zone inthe first embodiment has the design line indicated by L23 set byplotting the points Pλ1, Pλ2, and Pλ3 at the diffraction orders (k1 o,k2 o, k3 o)=(+4, +2, +2). In FIG. 49 the design point Pλ1 of the firstwavelength is positioned on the design line L23, so the aberration ofdiffraction light of the diffraction orders k1 o is approximately zero.On the other hand, the plotted points Pλ2 and Pλ3 of the second andthird wavelengths are significantly deviated from the aberration zerodesign point, indicating the above-described flaring. Note that in FIG.49, only (k2 o, k3 o)=(+2, +2) is shown plotted, but there is deviationfrom the design line L23 in the same way for other orders in the secondand third wavelengths as well. Consequently, there is uncorrectedaberration in the second wavelength, and consequently, the lightquantity of the optical beams of the second and third wavelengths whichhave passed through the outer ring zone, that is not imaged at thesignal recording face but input to the second and third optical discscan be suppressed. As a result, regardless of the diffraction efficiencyof the optical beam of the second wavelength as shown in FIG. 51, thisoptical beam does not contribute to image formation, and accordingly, asuitable aperture restriction (NA=0.6) can be realized. Also, regardlessof the diffraction efficiency of the optical beam of the thirdwavelength as shown in FIG. 51, this optical beam does not contribute toimage formation, and accordingly, a suitable aperture restriction(NA=0.45) can be realized.

As described above, with the outer ring zones in the first embodimentand a later-described second embodiment, the diffraction face is blazed,so according to this configuration, even in the case of providing thediffraction grooves to one face of the object lens as described later,diffraction grooves can be formed relatively easily at the curved faceof the lens face at the perimeter of the lens which has a steep slopedue to being at the outer ring zone.

Next, a second embodiment shown in Table 10 will be described.

TABLE 10 Diffraction efficiency, diffraction order, depth, and number ofsteps at each ring zone, of Second Embodiment K1 eff₁ K2 eff₂ K3 eff₃ d[μm] s Inner Ring zone 0 0.98 −1 0.78 −2 0.39 6.9 3 Middle Ring zone 00.96 −1 0.81 −3 ※ 11.65 5 Outer Ring zone 1 1.0 1 ※ 1 ※ 0.8 ∞ *represents that according to flaring, efficiency does not effect aproblem.

Also, with the inner ring zone according to the second embodiment, asshown in Table 10, when employing a staircase form with the number ofsteps S=3 and groove depth d=6.9 (μm), with the diffraction order k1 i=0of the optical beam of the first wavelength, the diffraction efficiencyis eff1=0.98, and with the diffraction order k2 i=−1 of the optical beamof the second wavelength, the diffraction efficiency is eff2=0.78, andwith the diffraction order k3 i=−2 of the optical beam of the thirdwavelength, the diffraction efficiency is eff3=0.39.

Next, description will be made further specifically regarding the innerring zone according to the second embodiment with reference to FIGS. 52Athrough 52C. FIG. 52A is a diagram illustrating change in thediffraction efficiency of zero-order diffracted light of the opticalbeam of the first wavelength in the case of changing the groove depth dof the staircase form of the number of steps S=3, FIG. 52B is a diagramillustrating change in the diffraction efficiency of −1st orderdiffracted light of the optical beam of the second wavelength in thecase of changing the groove depth d of the staircase form of the numberof steps S=3, and FIG. 52C is a diagram illustrating change in thediffraction efficiency of −2nd order diffracted light of the opticalbeam of the third wavelength in the case of changing the groove depth dof the staircase form of the number of steps S=3. In FIGS. 52A through52C, the horizontal axis represents groove depth (nm), and the verticalaxis represents diffraction efficiency (light intensity). At a positionwhere the horizontal axis is 6900 nm, as shown in FIG. 52A, eff1 is0.98, and as shown in FIG. 52B, eff2 is 0.78, and as shown in FIG. 52C,eff3 is 0.39.

Note that with the inner ring zone in the second embodiment as well, thediffraction orders (k1 i, k2 i, k3 i)=(0, −1, −2) selected here satisfythe above-mentioned conditional expression (1) (let us say that x of k1x, k2 x, and k3 x within the conditional expression is x=i), and arediffraction orders that can correct and reduce the spherical aberrationon the signal recording face of each optical disc. Further,specifically, as shown in FIG. 55, the respective plots Pλ1, Pλ2, andPλ3 are positioned in a straight line on the straight line L24 servingas a generally design line. Now, strictly, in the same way as describedabove with reference to FIG. 42, let us say that the second and thirdwavelengths λ2 and λ3 are input as divergent rays, thereby disposing ona straight line completely.

With the middle ring zone according to the second embodiment, as shownin Table 10, when employing a staircase form with the number of stepsS=5 and groove depth d=11.65 (μm), with the diffraction order k1 m=0 ofthe optical beam of the first wavelength, the diffraction efficiency iseff1=0.96, and with the diffraction order k2 m=−1 of the optical beam ofthe second wavelength, the diffraction efficiency is eff2=0.81. Also,the diffraction efficiency eff3 of the diffraction order k3 m=−3 servingas the maximum diffraction efficiency of the optical beam of the thirdwavelength transmitting this region is around 0.4, but this does notcontribute to image formation since the spot is subjected to flaring asdescribed above (see FIG. 56).

Next, description will be made further specifically regarding the middlering zone according to the second embodiment with reference to FIGS. 53Athrough 53C. FIG. 53A is a diagram illustrating change in thediffraction efficiency of zero-order diffracted light of the opticalbeam of the first wavelength in the case of changing the groove depth dof the staircase form of the number of steps S=5, FIG. 53B is a diagramillustrating change in the diffraction efficiency of −1st orderdiffracted light of the optical beam of the second wavelength in thecase of changing the groove depth d of the staircase form of the numberof steps S=5, and FIG. 53C is a diagram illustrating change in thediffraction efficiency of −3rd order diffracted light of the opticalbeam of the third wavelength in the case of changing the groove depth dof the staircase form of the number of steps S=5. In FIGS. 53A through53C, the horizontal axis represents groove depth (nm), and the verticalaxis represents diffraction efficiency (light intensity). At a positionwhere the horizontal axis is 11650 nm, as shown in FIG. 53A, eff1 is0.96, and as shown in FIG. 53B, eff2 is 0.81, and as shown in FIG. 53C,eff3 is around 0.4, but the spot is subjected to flaring.

Also, with the middle ring zone in the second embodiment, in the sameway as the case of the middle ring zone in the first embodimentdescribed above, an arrangement is made wherein the design line of theobject lens is deviated, and flaring is carried out regarding the thirdwavelength to perform excellent aperture restriction. Specifically, asshown in FIG. 56, the middle ring zone in the second embodiment has thedesign line indicated by L25 set by plotting the points Pλ1, Pλ2, andPλ3 at the diffraction orders (k1 m, k2 m, k3 m)=(0, −1, −3). In FIG. 56the design point Pλ1 of the first wavelength and the design point Pλ2 ofthe second wavelength are positioned on the design line L25, so theaberration of diffraction light of the diffraction orders k1 m and k2 mis approximately zero. On the other hand, the plotted point Pλ3 of thethird wavelength is significantly deviated from the aberration zerodesign point, indicating the above-described flaring. Note that in FIG.56, only k3 m=−3 is shown plotted, but there is deviation from thedesign line L25 in the same way for other orders in the third wavelengthas well. Consequently, there is uncorrected aberration in the thirdwavelength, and consequently, the light quantity of the optical beam ofthe third wavelength which has passed through the middle ring zone, thatis not imaged at the signal recording face but input to the thirdoptical disc can be suppressed. As a result, regardless of thediffraction efficiency of the optical beam of the third wavelength asshown in FIG. 53, these optical beams do not contribute to imageformation, and accordingly, a suitable aperture restriction (NA=0.45)can be realized.

With the outer ring zone according to the second embodiment, as shown inTable 10, when employing a blazed form (S=∞) with groove depth d=0.8(μm), with the diffraction order k1 o=+1 of the optical beam of thefirst wavelength, the diffraction efficiency is eff1=1.0. Also, with thediffraction order k2 o=+1 serving as the maximum diffraction efficiencyof the optical beam of the second wavelength transmitting this region,the diffraction efficiency eff2 is around 0.6, but this does notcontribute to image formation since the spot is subjected to flaring asdescribed above (see FIG. 57). Further, the diffraction efficiency eff3of the diffraction order k3 o=+1 serving as the maximum diffractionefficiency of the optical beam of the third wavelength transmitting thisregion is around 0.4, but this does not contribute to image formationsince the spot is subjected to flaring as described above.

Next, description will be made further specifically regarding the outerring zone according to the second embodiment with reference to FIGS. 54Athrough 54C. FIG. 54A is a diagram illustrating change in thediffraction efficiency of +1st order diffracted light of the opticalbeam of the first wavelength in the case of changing the groove depth dof the blazed form of the number of steps S=∞, FIG. 54B is a diagramillustrating change in the diffraction efficiency of +1st orderdiffracted light of the optical beam of the second wavelength in thecase of changing the groove depth d of the blazed form of the number ofsteps S=∞ . . . and also illustrating change in diffraction efficiencyof zero order light which is unwanted light, and FIG. 54C is a diagramillustrating change in the diffraction efficiency of +1st orderdiffracted light of the optical beam of the third wavelength in the caseof changing the groove depth d of the blazed form of the number of stepsS=∞ . . . and also illustrating change in diffraction efficiency of zeroorder light which is unwanted light. In FIGS. 54A through 54C, thehorizontal axis represents groove depth (nm), and the vertical axisrepresents diffraction efficiency (light intensity). At a position wherethe horizontal axis is 800 nm, as shown in FIG. 54A, eff1 is 1.0, and asshown in FIG. 54B, eff2 is around 0.6, but the spot is subjected toflaring, and as shown in FIG. 54C, eff3 is around 0.4, but the spot issubjected to flaring.

Also, with the outer ring zone in the second embodiment described above,in the same way as the case of the outer ring zone in the firstembodiment described above, an arrangement is made wherein the designline of the object lens is deviated, and flaring is carried outregarding the second and third wavelengths to perform excellent aperturerestriction. Specifically, as shown in FIG. 57, the outer ring zone inthe second embodiment has the design line indicated by L26 set byplotting the points Pλ1, Pλ2, and Pλ3 at the respective diffractionorders (k1 o, k2 o, k3 o)=(+1, +1, +1). In FIG. 57 the design point Pλ1of the first wavelength is positioned on the design line L26, so theaberration of diffraction light of the diffraction order k1 o isapproximately zero. On the other hand, the plotted points Pλ2 and Pλ3 ofthe second and third wavelengths are significantly deviated from theaberration zero design point, indicating the above-described flaring.Note that in FIG. 57, only (k2 o, k3 o)=(+1, +1) is shown plotted, butthere is deviation from the design line L26 in the same way for otherorders, such as zero order light for example, in the second and thirdwavelengths as well. Consequently, there is uncorrected aberration inthe second and third wavelengths, and consequently, the light quantityof the optical beams of the second and third wavelengths which havepassed through the outer ring zone, that is not imaged at the signalrecording face but input to the second and third optical discs can besuppressed. As a result, regardless of the diffraction efficiency of theoptical beam of the second wavelength as shown in FIG. 54, this opticalbeam does not contribute to image formation, and accordingly, a suitableaperture restriction (NA=0.6) can be realized. Also, regardless of thediffraction efficiency of the optical beam of the third wavelength asshown in FIG. 54, this optical beam does not contribute to imageformation, and accordingly, a suitable aperture restriction (NA=0.45)can be realized.

The diffraction units according to the first and second embodimentshaving such an inner ring zone, middle ring zone, and outer ring zone,the relation of the above-mentioned Expression (5B) is satisfied,diffraction efficiency as to the respective wavelengths is excellent forall ring zones, whereby sufficient efficiency can be obtained, and itcan be confirmed that the problem of unwanted light is eliminated. Also,as described above, the inner ring zone is formed in a step form(staircase form), and the outer ring zone is formed in a blazed form,which is an advantageous configuration on manufacturing as well.

Next, the first and second embodiments are confirmed from theperspective of operating distance and focal distance. Each wavelength ofthe first and second embodiments shown in Tables 9 and 10, and theoptical properties as to the corresponding optical disc are shown in thefollowing Tables 11 and 12. Note that Table 11 corresponds to the firstembodiment, and Table 12 corresponds to the second embodiment. Also,Tables 11 and 12 show “focal distance”, “NA”, “effective diameter”,“magnification”, and “operating distance” of the object lens, as to theoptical beam of each wavelength and the corresponding optical disc,“thickness of protective layer” of the optical disc, and “axialthickness” of the object lens.

TABLE 11 Optical properties as to each disc and corresponding eachwavelength, of First Embodiment First optical Second optical Thirdoptical disc disc disc λ1 λ2 λ3 Focal distance [mm] 2.2 2.28 2.30 NA0.85 0.60 0.45 Effective diameter [mm] 3.74 2.72 2.07 Magnification 0 −1/60 − 1/60 Thickness of protective 0.0875 0.6 1.1 layer [mm] Operatingdistance [mm] 0.92 0.70 0.41 Axial thickness [mm] 2.13

TABLE 12 Optical properties as to each disc and corresponding eachwavelength, of Second Embodiment First optical Second optical Thirdoptical disc disc disc λ1 λ2 λ3 Focal distance [mm] 1.92 2.05 2.19 NA0.85 0.60 0.45 Effective diameter [mm] 3.26 2.46 2.00 Magnification 0 −1/50 − 1/60 Thickness of protective 0.0875 0.6 1.1 layer [mm] Operatingdistance [mm] 0.62 0.52 0.40 Axial thickness [mm] 2.20

As shown in Tables 11 and 12, with the diffraction units according tothe first and second embodiments, “focal distance” as to the firstwavelength can be suppressed to equal to or smaller than 2.2, and“operating distance” in the case of employing the optical beam of thethird wavelength can be set to equal to or greater than 0.40, which arerequired as described above.

As described above, with the diffraction units according to the firstand second embodiments, the configurations advantageous to manufacturingcan be provided, the problem of unwanted light can be eliminated, theconditions for the focal distance and operating distance of the objectlens as to each wavelength can be set desirably, and predeterminedaperture restriction and desired diffraction efficiency can be obtainedas to each wavelength.

Note that description has been made above assuming that there areprovided the first diffraction region 251 where the diffractionstructure of the staircase form is formed wherein staircase structuresincluding multiple step portions as inner ring zones are consecutivelyformed in the radial direction of the ring zones, the second diffractionregions 252 and 252B where the diffraction structure of the staircaseform or blazed form is formed wherein staircase structures includingmultiple step portions as middle ring zones are consecutively formed inthe radial direction of the ring zones, and the third diffraction region253 where the diffraction structure of the blazed form is formed as anouter ring zone, but the present invention is not restricted to this, sothe inner ring zones and middle ring zones may be configured of thediffraction structure which is an non-cyclical structure as long as thisstructure satisfies the above-mentioned relation of a diffraction orderto be selected.

For example, the first diffraction region may be configured such that anon-cyclical diffraction structure is formed, wherein an non-cyclicalstructure for providing desired phase difference is formed in the radialdirection of the ring zones as described above, and the seconddiffraction region may be configured such that a non-cyclicaldiffraction structure is formed, wherein an non-cyclical structure forproviding desired phase difference is formed in the radial direction ofthe ring zones as described above. In the case of providing anon-cyclical diffraction structure in the first and second diffractionregions, flexibility of design is extended, more desirable diffractionefficiency can be obtained, which is an advantageous structure from theperspective of the temperature properties of diffraction efficiency.

Also, as a modification of the above-mentioned first through thirddiffraction regions 251, 252, and 253, the third diffraction region maybe formed as a so-called aspherical continuous face. Specifically, anarrangement may be made wherein predetermined refractive power isapplied to the optical beam of the first wavelength by the refractivepower of a lens curved face instead of such a third diffraction region253 such as described above to condense the optical beam on thecorresponding optical disc in a state wherein there is no sphericalaberration, and the optical beams of the second and third wavelengthsare subjected to aperture restriction suitably. In other words, thediffraction unit may be configured as a diffraction unit including thefirst diffraction region 251 where the diffraction structure of thestaircase form is formed wherein staircase structures, formed on aregion corresponding to the numerical aperture of the third opticaldisc, including multiple step portions as inner ring zones areconsecutively formed in the radial direction of the ring zones, thesecond diffraction regions 252 and 252B where the diffraction structureof the staircase form or blazed form is formed wherein staircasestructures, formed on a region corresponding to the numerical apertureof the second optical disc, including multiple step portions as middlering zones are consecutively formed in the radial direction of the ringzones, and a region formed on a region corresponding to the numericaperture of the first optical disc wherein the optical beam of the firstwavelength transmitted therethrough is condensed on the signal recordingface of the corresponding first optical disc, and the optical beams ofthe second and third wavelengths transmitted therethrough are notcondensed on the signal recording faces of the corresponding second andthird optical discs.

With the diffraction unit 250 including the first through thirddiffraction regions 251, 252, and 253 thus configured, the optical beamsof the first through third wavelengths transmitted through the firstdiffraction region 251 can be diffracted by diffraction power so as togenerate a divergent angle state wherein no spherical aberration occurson the signal recording face of the corresponding type of optical discby the refractive power of the object lens 234 which is common to thethree wavelengths, a suitable spot can be condensed on the signalrecording face of the corresponding optical disc by the refractive powerof the object lens 234, the optical beams of the first and secondwavelengths transmitted through the second diffraction region 252 can bediffracted by diffraction power so as to generate a divergent anglestate wherein no spherical aberration occurs on the signal recordingface of the corresponding type of optical disc by the refractive powerof the common object lens, a suitable spot can be condensed on thesignal recording face of the corresponding optical disc by therefractive power of the object lens 234, the optical beam of the firstwavelength transmitted through the third diffraction region 253 can bediffracted by diffraction power so as to generate a divergent anglestate wherein no spherical aberration occurs on the signal recordingface of the corresponding type of optical disc by the refractive powerof the object lens 234, and a suitable spot can be condensed on thesignal recording face of the corresponding optical disc by therefractive power of the object lens 234. Here, “a divergent angle statewherein no spherical aberration occurs” includes a diverged state,converged state, and parallel light state, and means a state whereinspherical aberration is corrected by the refractive power of a lenscurved face.

That is to say, with the diffraction unit 250 provided on one face ofthe object lens 234 disposed on the optical path between the firstthrough third emitting units of the optical system the optical pickup203 and the signal recording face, diffraction power can be applied tooptical beams of respective wavelengths passing through the respectiveregions (first through third diffraction regions 251, 252, and 253) soas to be in a state wherein spherical aberration occurring at the signalrecording face is reduced, so spherical aberration occurring at thesignal recording face when condensing optical beams of the first throughthird wavelengths on the signal recording face of the respectivecorresponding optical discs using the common object lens 234 in theoptical pickup 203 can be minimized, which is to say thatthree-wavelength compatibility of the optical pickup 203 using threetypes of wavelengths for three types of optical discs and the commonobject lens 234 can be realized, wherein information signals can berecorded to and/or played from respective optical discs.

Also, the object lens 234 having the diffraction unit 250 configured ofthe first through third diffraction regions 251, 252, and 253 asdescribed above is configured having the relation k1 i≧k2 i>k3 i for thediffraction orders (k1 i, k2 i, k3 i) selected by the first diffractionregion 251 serving as the inner ring zone so as to be dominant andcondensed on the signal recording face of the corresponding optical discvia the object lens 234, so making diffracted light of an orderregarding which spherical aberration can be suitably reduced dominant,enables a suitable spot to be condensed on the signal recording face ofthe optical discs corresponding to the optical beams of each wavelength,and realize a suitable state for the operating distance for using theoptical beams of each wavelength and a focal distance for eachwavelength, which is to say in the case of using the third wavelength λ3the focal distance can be prevented from becoming too long as to thefirst wavelength λ1 in order to ensure operating distance thereof,thereby preventing problems such as the lens diameter of the object lensbeing large, the overall size of the optical pickup being large, and soforth. Accordingly, the object lens 234 having the diffraction unit 250realizes condensing optical beams of each wavelength to form a suitablespot on the signal recording face of the corresponding optical discswith high light use efficiency without increasing the size of theoptical parts and optical pickup by ensuring a suitable operatingdistance and focal distance, which is to say that three-wavelengthcompatibility of the optical pickup using three types of wavelengths forthree types of optical discs and the common object lens 234 can berealized, wherein information signals can be suitably recorded to and/orplayed from respective optical discs.

Also, the object lens 234 having the diffraction unit 250 such asdescribed above is configured such that, of the diffraction ordersselected by the first diffraction region 251 serving as the inner ringzone so as to be dominant and condensed on the signal recording face ofthe corresponding optical disc via the object lens, k1 i and k3 i are(−2, −3), (−1, −2), (−1, −3), (0, −2), (0, −3), (1, −2), (1, −3), (2,−1), (2, −2), (2, −3), (3, 0), (3, −1), (3, −2), or (3, −3), so makingdiffracted light of an order regarding which spherical aberration can besuitably reduced enables a suitable spot to be condensed on the signalrecording face of the optical discs corresponding to the optical beamsof each wavelength, and realize a suitable state for the operatingdistance for using the optical beams of each wavelength and a focaldistance for each wavelength, which is to say in the case of using thethird wavelength λ3 the focal distance can be prevented from becomingtoo long as to the first wavelength λ1 in order to ensure operatingdistance thereof, thereby preventing problems such as the lens diameterof the object lens being large, the overall size of the optical pickupbeing large, and so forth, and additionally, as described above withregard to the third perspective for configuring the inner ring zone, theconfiguration is advantageous from a manufacturing viewpoint in that thenecessary depth of the grooves is prevented from becoming too deep,whereby the manufacturing process can be simplified, and alsodeterioration of forming precision can be prevented. Accordingly, theobject lens 234 having the diffraction unit 250 can realize condensingoptical beams of each wavelength on the signal recording face of thecorresponding optical discs to form a suitable spot with high light useefficiency without increasing the size of the optical parts and opticalpickup by ensuring a suitable operating distance and focal distance, andalso simplifies manufacturing process and prevents deterioration offorming precision.

Also, the object lens 234 having the diffraction unit 250 such asdescribed above is configured such that the first diffraction region 251has formed a staircase form diffraction structure wherein a staircasestructure with multiple steps continue in the radial direction of thering zones, and the third diffraction region 253 has a blazeddiffraction structure formed. The object lens 234 having the diffractionunit 250 has the inner ring zone, which needs to provide the firstthrough third wavelengths with a diffraction power so as to be in apredetermined state, and also have high diffraction efficiency, formedin a stepped shape, thereby suppressing the quantity of diffracted lightof unwanted light, preventing deterioration of jittering and the likedue to unwanted light being received at the photosensor, and also, evenin cases of a certain amount of diffracted light of unwanted lightoccurring, unwanted light being received at the time of focusing leadingto deterioration of jittering and the like can be prevented by makingthe diffraction order of the unwanted light to be a deviated order withgreat diffraction angle difference, that is other than an adjacentdiffraction order of the focus light. Also, the object lens 234 havingthe diffraction unit 250 has a configuration has the outer ring zoneprovided integrally on one face of the object lens and also provided onthe outermost side thereof, formed in a blazed form, which is anadvantageous structure in the case of forming a diffraction structure atportions having an extremely steep lens curved surface, such as with athree-wavelength-compatible lens, whereby manufacturing can befacilitated and deterioration in forming precision can be prevented.

Also, the object lens 234 having the diffraction unit 250 such asdescribed above is configured such that the optical beam of the firstwavelength at the time of input to the incident side of the object lens234 is an infinite optical system, i.e., generally parallel light, andthe optical beams of the second and third wavelengths are input as afinite optical system, i.e., as divergent light, whereby, as describedwith reference to FIGS. 41, 42, and 55, optical beams passing throughthe first diffraction region 251 which is the inner ring zone wherethere is need to take into consideration the possibility of sphericalaberration correction can be suitably condensed on the signal recordingface of the optical disc in a state of high diffraction efficiency andno spherical aberration as predetermined diffraction efficiency as tothe selected diffraction orders k1 i, k2 i, and k3 i for the threewavelengths. Further, due to the configuration wherein the optical beamof the first wavelength at the time of input to the incident side of theobject lens is generally parallel light and the optical beams of thesecond and third wavelengths are input as divergent light, the objectlens 234 having the diffraction unit 250 has improved freedom forflaring at the middle ring zone and outer ring zone as described withreference to FIGS. 48, 49, 56, and 57, and by improving freedom andbenefiting from the advantages of flaring, the freedom of diffractionstructure selection of the middle ring zone and outer ring zone isimproved, i.e., higher efficiency can be obtained, and also thestricture itself is simplified, further enabling deterioration information precision thereof to be prevented. Thus, due to theconfiguration wherein the optical beam of the first wavelength at thetime of input to the incident side of the object lens 234 is generallyparallel light and the optical beams of the second and third wavelengthsare input as divergent light, the object lens 234 having the diffractionunit 250 can realize suitably condensing light of each wavelength at thesignal recording face of the corresponding optical disc in a state ofhigh diffraction efficiency and no spherical aberration, with a simplerconfiguration.

Note that in the event that the diffraction unit 250 is to be providedto a diffraction optical element 235B separate from the object lens asdescribed later (see FIG. 58B), the same advantages can be had with aconfiguration wherein, of the object lens and the diffraction opticalelement to which the diffraction unit has been provided, the elementpositioned at the side closer to the first through third emitting unitsis configured such that the optical beam of the first wavelength at thetime of input to the incident side thereof is generally parallel lightand the optical beams of the second and third wavelengths are input asdivergent light.

Further, the object lens 234 having the diffraction unit 250 such asdescribed above is configured such that the diffraction orders (k1 i, k2i, k3 i) of light selected by the first diffraction region 251 servingas the inner ring zone and made dominant, and condensed onto the signalrecording face of the corresponding optical disc via the object lens234, are (1, −1, −2), (0, −1, −2), (1, −2, −3) or (0, −2, −3), wherebyspherical aberration at each wavelength described with respect to thefirst perspective can be reduced when configuring the inner ring zone,the operating distance and focal distance at each wavelength describedwith respect to the second perspective can be made optimal, aconfiguration which is advantageous from the aspect of manufacturing asdescribed with respect to the third and fourth perspectives can berealized, and further, the diffraction efficiency of the diffractionorders selected for each wavelength can be set sufficiently high, andalso diffraction efficiency of unwanted light can be suppressed due toenabling configuration with the stepped form, so adverse effects ofunwanted light can be maximally suppressed since the diffractionefficiency of the adjacent diffraction order can be suppressed to a lowlevel. Accordingly, the object lens 234 having the diffraction unit 250realizes condensing light for a suitable spot on the signal recordingface of corresponding optical discs with high light use efficiency,using a more advantageous configuration taking into consideration a morespecific configuration and the advantages of reduction in size and ofthe configuration.

Further, with the object lens 234 having the diffraction unit 250 suchas described above, when the diffraction orders (k1 i, k2 i, k3 i) oflight selected by the first diffraction region 251 serving as the innerring zone are as above, the diffraction orders (k1 m, k2 m) of lightselected by the second diffraction region 252 serving as the middle ringzone and made dominant, and condensed onto the signal recording face ofthe corresponding optical disc via the object lens 234, are (+1, +1),(−1, −1), (0, +2), (0, −2), (0, +1), (0, −1), (+1, 0), or (−1, 0),whereby a configuration can be realized in a staircase form ornon-cyclical form diffraction structure which is advantageous regardingdiffraction efficiency for example, whereby the functions of the innerring zone and middle ring zone can be each sufficiently manifested. Thatis to say, the object lens 234 having the second diffraction region 252configured thus is of a configuration wherein, at the time ofconfiguring the middle ring zone in particular, matching the image pointposition with the diffraction functions at the inner ring zone andmiddle ring zone such as described with respect to the secondperspective is easier, so optical beams of the first and secondwavelengths input to the middle ring zone can be placed in a state wherethe relation with the optical beam of which aberration has been reducedas described above by the inner ring zone is optimal, and also sphericalaberration can be sufficiently reduced. Further, with the object lens234 having the second diffraction region 252, high diffractionefficiency can be obtained regarding the first and second wavelengths ina state of spherical aberration having been corrected, and also suitableaperture restriction can be performed regarding the third wavelength,and also the configuration is advantageous from a manufacturingviewpoint. Accordingly, the object lens 234 having the diffraction unit250 realizes condensing a suitable spot on the signal recording face ofthe corresponding optical disc with high light use efficiency, with amore advantageous configuration taking into consideration advantages ofconfiguration and so forth.

Further, with the object lens 234 having the diffraction unit 250 suchas described above, when the diffraction orders (k1 i, k2 i, k3 i) oflight selected by the first diffraction region 251 serving as the innerring zone are as above, the diffraction orders (k1 m, k2 m) of lightselected by the second diffraction region 252 serving as the middle ringzone and made dominant, and condensed onto the signal recording face ofthe corresponding optical disc via the object lens 234, are (+3, +2),(−3, −2), (+2, +1), or (−2, −1), whereby a configuration can be realizedin a blazed form or non-cyclical form diffraction structure which isadvantageous regarding diffraction efficiency for example, whereby thefunctions of the inner ring zone and middle ring zone can be eachsufficiently manifested. That is to say, the object lens 234 having thesecond diffraction region 252 configured thus is of a configurationwherein, at the time of configuring the middle ring zone in particular,matching the image point position with the diffraction functions at theinner ring zone and middle ring zone such as described with respect tothe second perspective is easier, so optical beams of the first andsecond wavelengths input to the middle ring zone can be placed in astate where the relation with the optical beam of which aberration hasbeen reduced by the inner ring zone as described above is optimal, andalso spherical aberration can be sufficiently reduced. Further, with theobject lens 234 having the second diffraction region 252, highdiffraction efficiency can be obtained regarding the first and secondwavelengths in a state of spherical aberration having been corrected,and also suitable aperture restriction can be performed regarding thethird wavelength, and also the configuration is advantageous from amanufacturing viewpoint. Accordingly, the object lens 234 having thediffraction unit 250 realizes condensing a suitable spot on the signalrecording face of the corresponding optical disc with high light useefficiency, with a more advantageous configuration taking intoconsideration advantages of configuration and so forth.

Further, with the object lens 234 having the diffraction unit 250 suchas described above, when the diffraction orders (k1 i, k2 i, k3 i) oflight selected by the first diffraction region 251 serving as the innerring zone are as above, the diffraction orders (k1 m, k2 m) of lightselected by the second diffraction region 252 serving as the middle ringzone and made dominant, and condensed onto the signal recording face ofthe corresponding optical disc via the object lens 234, are (+1, −1), or(−1, +1), whereby a configuration can be realized in a staircase form ornon-cyclical form diffraction structure which is advantageous regardingdiffraction efficiency for example, and also (k1 m, k2 m) are (+1, +1),or (−1, −1), whereby a configuration can be realized in a blazed form ornon-cyclical form diffraction structure which is advantageous regardingdiffraction efficiency for example, whereby the functions of the innerring zone and middle ring zone can be each sufficiently manifested. Thatis to say, the object lens 234 having the second diffraction region 252configured thus is of a configuration wherein, due to being used alongwith a configuration wherein the effects of unwanted light are reducedby a technique such as setting the return power or the optical pickupoptical system higher, at the time of configuring the middle ring zonein particular, matching the image point position with the diffractionfunctions at the inner ring zone and middle ring zone such as describedwith respect to the second perspective is easier, so optical beams ofthe first and second wavelengths input to the middle ring zone can beplaced in a state where the relation with the optical beam of whichaberration has been reduced by the inner ring zone as described above isoptimal, and also spherical aberration can be sufficiently reduced.Further, with the object lens 234 having the second diffraction region252, high diffraction efficiency can be obtained regarding the first andsecond wavelengths in a state of spherical aberration having beencorrected, and also suitable aperture restriction can be performedregarding the third wavelength, and also the configuration isadvantageous from a manufacturing viewpoint. Accordingly, the objectlens 234 having the diffraction unit 250 realizes condensing a suitablespot on the signal recording face of the corresponding optical disc withhigh light use efficiency, with a more advantageous configuration takinginto consideration advantages of configuration and so forth.

Also, the diffraction unit 250 having the first through thirddiffraction regions 251, 252, and 253 is configured such that theoptical beam of the third wavelength passing through the second andthird diffraction regions 252 and 253 results in the diffracted light ofa diffraction order output with maximum diffraction efficiency and apredetermined diffraction efficiency being flared and the imagingposition is shifted from the signal recording face, thereby reducing thediffraction efficiency of the diffracted light of the diffraction order,whereby, with regard to the optical beam of the third wavelength, onlythe portion of the optical beam which has passed through the firstdiffraction region 251 is condensed on the signal recording face of theoptical disc by the object lens 234, and the first diffraction region251 is formed to a size such that the optical beam of the thirdwavelength passing through this region is shaped to have a size of apredetermined numerical aperture, whereby aperture restriction can beperformed regarding the optical beam of the third wavelength so as tohave a numerical aperture of around 0.45, for example.

Also, the diffraction unit 250 is configured such that that the opticalbeam of the second wavelength passing through the third diffractionregions 253 results in the diffracted light of a diffraction orderoutput with maximum diffraction efficiency and a predetermineddiffraction efficiency being flared and the imaging position is shiftedfrom the signal recording face, thereby reducing the diffractionefficiency of the diffracted light of the diffraction order, whereby,with regard to the optical beam of the second wavelength, only theportion of the optical beam which has passed through the first andsecond diffraction regions 251 and 252 is condensed on the signalrecording face of the optical disc by the object lens 234, and the firstand second diffraction regions 251 and 252 are formed to a size suchthat the optical beam of the second wavelength passing through thisregion is shaped to have a size of a predetermined numerical aperture,whereby aperture restriction can be performed regarding the optical beamof the second wavelength so as to have a numerical aperture of around0.60, for example.

Also, the diffraction unit 250 performs places the optical beam of thefirst wavelength passing outside of the third diffraction region 253 ina state so as to not be suitably condensed on the signal recording faceof the corresponding type of optical disc via the object lens 234, orshields the optical beam of the first wavelength passing outside of thethird diffraction region 253, whereby, with regard to the optical beamof the first wavelength, only the optical beam portion which has passedthrough the first through third diffraction regions 251, 252, and 253 iscondensed on the signal recording face of the optical disc via theobject lens 234, and also, the first through third diffraction regions251, 252, and 253 are formed to a size which is the predeterminednumerical aperture of the first wavelength optical beam passing throughthis region, whereby aperture restriction can be performed regarding theoptical beam of the first wavelength such that NA=around 0.85, forexample.

Thus, the diffraction unit 250 provided on one face of the by the objectlens 234 disposed on the optical path as described above not onlyrealizes three-wavelength compatibility, but also enables optical beamsof each wavelength to be input to the common object lens 234 in a statewherein aperture restriction is performed with a numerical apertureappropriate for each of the three types of optical discs and opticalbeams of the first through third wavelengths, thereby not only havingfunctions of aberration correction corresponding to the threewavelengths, but also serving as an aperture restricting unit.

It should be noted that a diffraction unit can be configured by suitablycombining the diffraction regions in the above-described embodiments.That is to say, the diffraction order of each wavelength passing througheach diffraction region can be selected as appropriate. In the event ofchanging the diffraction order of each wavelength passing through eachdiffraction region, an object lens 234 having a lens curve facecorresponding to each diffraction order of each wavelength passingthrough each diffraction region can be used.

Also, while description has been made above with the diffraction unit250 configured of the three diffraction regions 251, 252, and 253 formedon the incident side face of the object lens 234, as shown in FIG. 58A,the present invention is not restricted to this arrangement, and may beprovided to the output side of the object lens 234. Further, thediffraction unit 250 having the first through third diffraction regions251, 252, and 253, can be integrally configured on the input or outputside of an optical element provided separately from the object lens 234,and as shown in FIG. 58B for example, a condensing optical device may beconfigured including an object lens 234B which has only a lens curvewith the diffraction unit 250 removed therefrom, and a diffractionoptical element 235B with the diffraction unit 250 provided on one facethereof and disposed on the optical path shared by the threewavelengths. With the object lens 234 shown in FIG. 58A for example, theplanar shape of a diffraction structure required for the functions ofdiffractive power is combined with a reference face at the incident siderequired for the lens to be able to have functions of refractive power,conversely, in the case shown in FIG. 58B wherein a separate diffractionoptical element 235B is provided, the object lens 234B itself has theplanar shape of a diffraction structure required for the functions ofrefractive power, and the diffraction optical element 235B has formed onone face thereof a diffraction structure required for the functions ofdiffractive power. The object lens 234B and diffraction optical element235B shown in FIG. 58B function as a condensing optical device in thesame way as the above-described object lens 234, so as to reduceaberration and the like and also realize three-wavelength compatibilityof the optical pickup due to being used as the optical pickup andmanifests advantages of enabling further reduction in optical parts andalso simplification of configuration and reduction in configurationsize, high productivity, and low costs, and the diffraction structurecan be made more complex as compared with a case of integrally providingon the object lens 234. On the other hand, the arrangement shown in FIG.58A described above functioning as a condensing optical device whichsuitably condenses the optical beams of three different wavelengths onthe signal recording face of respectively corresponding optical discssuch that spherical aberration does not occur, with the single element(object lens 234) alone configured of the diffraction unit 250integrally provided to the object lens 234, enables further reduction inoptical parts and reduction in size of the configuration. Note that theabove-described diffraction unit 250 sufficiently manifests theadvantages thereof with the diffraction structure for aberrationcorrection to realize three-wavelength compatibility being provided on asingle face that has been difficult with the related art, which enablessuch a refractive element to be integrally formed with the object lens234, further enabling directly forming a diffraction face on a plasticlens, and forming the object lens 234 with which the diffraction unit250 has been integrated of a plastic material further realizes improvedproduction and lower costs.

The collimator lens 242 provided between the object lens 234 and thethird beam splitter 238 converts the divergent angle of each of thefirst through third wavelength optical beams of which the optical pathshave been synthesized at the second beam splitter 237 and passed throughthe third beam splitter 238, and outputs to the quarter-wave plate 243and object lens 234 side, in a generally parallel light state, forexample. The arrangement wherein the collimator lens 242 inputs theoptical beams of the first and second wavelengths into theabove-described object lens 234 with the divergent angle thereof in thestate of generally parallel light, and also inputs the optical beam ofthe third wavelength into the object lens 234 in a state which isslightly diffused or converged as to parallel light (hereinafter alsoreferred to as “finite system state”) enables further reduction ofspherical aberration at the time of condensing the second and thirdwavelength optical beams on the signal recording face of the second andthird optical discs via object lens 234, realizing three-wavelengthcompatibility with even less aberration occurring. This point has beendescribed above with reference to FIGS. 41 and 42. While an arrangementhas been described here wherein the optical beam of the third wavelengthis input to the object lens 234 in a state of a predetermined divergentangle, due to the positional relation between the second light source232 having the second emitting unit for emitting the second wavelengthoptical beam and the collimator lens 242, and/or the positional relationbetween the third light source 233 having the third emitting unit foremitting the third wavelength optical beam and the collimator lens 242,in the event of positioning multiple emitting units at a common lightsource for example, this may be realized by providing an element whichconverts only the divergent angle of the optical beam of the secondand/or third wavelengths, or by inputting into the object lens 234 in apredetermined divergent angle state by providing a mechanism to drivethe collimator lens 242 or the like. Also, either the optical beams ofthe second wavelength, or the optical beams of the second and thirdwavelengths, may be input to the object lens 234 in the finite systemstate in accordance with the situation, thereby further reducingaberration. Also, optical beams of the second and third wavelengths maybe input in the finite system state and in a diffused state, therebyrealizing adjustment of return power and even more excellent opticalsystem compatibility may be achieved by setting the focus capture rangeand so forth to a desired state matching the format by adjusting thereturn power.

The multi-lens 246 is, for example, a wavelength-selective multi-lens,whereby the returning first through third wavelength optical beamsseparated from the outgoing path optical beams by being reflected at thethird beam splitter 238, after having been reflected off of the signalrecording face of the respective optical disc, and passed through theobject lens 234, redirecting mirror 244, quarter-wave plate 243, andcollimator lens 242, is appropriately condensed on the photoreceptionface of the photodetector or the like of the photosensor 245. At thistime, the multi-lens 246 provides the return optical beam withastigmatism for detection of focus error signals or the like.

The photosensor 245 receives the return optical beam condensed at themulti-lens 246, and detects, along with information signals, varioustypes of detection signals such as focus error signals, tracking errorsignals, and so forth.

With the optical pickup 203 configured as described above, the objectlens 234 is driven so as to be displaced based on the focus errorsignals and tracking error signals obtained by the photosensor 245,whereby the object lens 234 is moved to a focal position as to thesignal recording face of the optical disc 2, the optical beam is focusedonto the signal recording face of the optical disc 2, and information isrecorded to or played from the optical disc 2.

The optical pickup 203 is provided on one face of the object lens 234,can provide optical beams of each wavelength with a diffractionefficiency and diffraction angle suitable for each region due to thediffraction unit 250 having the first through third diffraction regions251, 252, and 253, can sufficiently reduce spherical aberration at thesignal recording face of the three types of first through third opticaldiscs 11, 12, and 13, of which the format for the thickness of theprotective layer or the like differs, and enables reading and writing ofsignals to and from the multiple types of optical discs 11, 12, and 13,using optical beams of three different wavelengths.

Also, the object lens 234 having the diffraction unit 250 shown in FIG.58A, and the diffraction optical element 235B having the diffractionunit 250 and object lens 234B described with reference to FIG. 58B,making up the above-described optical pickup 203, can each function as acondensing optical device for condensing input optical beams atpredetermined positions. In the event of using this condensing opticaldevice for an optical pickup which performs recording and/or playing ofinformation signals by irradiating optical beams onto three differenttypes of optical discs, the diffraction unit 250 provided on one face ofthe object lens 234 or the diffraction optical element 235B enables thecondensing optical device to appropriately condense correspondingoptical beams onto the signal recording face of the three types ofoptical discs in a state with spherical aberration sufficiently reduced,meaning that three-wavelength compatibility of the optical pickup usingthe object lens 234 or the object lens 234B common to the threewavelengths can be realized.

Also, while the diffraction optical element 235B having the diffractionunit 250 and object lens 234B described with reference to FIG. 58B forexample may be provided to an actuator such as an object lens drivingmechanism or the like for driving the object lens 234B such that thediffraction optical element 235B having the diffraction unit 250 and theobject lens 234B are integral, this may be configured as a condensingoptical unit wherein the diffraction optical element 235B and objectlens 234B are formed as an integrated unit, in order to improveprecision of assembly to the lens holder of the actuator, and facilitateassembly work. For example, a condensing optical unit can be configuredby using spacers or the like to fix the diffraction optical element 235Band object lens 234B to the holder while setting the positioning,spacing, and optical axis, so as to be integrally formed. Due to beingintegrally assembled to the object lens driving mechanism as describedabove, the diffraction optical element 235B and object lens 234B canappropriately condense the first through third wavelength optical beamson the signal recording face of the respective optical discs withspherical aberration reduced, even at the time of field shift such asdisplacement in the tracking direction, and so forth, for example.

Next, the optical paths of the optical beams emitted from the firstthrough third light sources 231, 232, and 233 of the optical pickup 203configured as described above, will be described with reference to FIG.37. First, the optical path at the time of emitting the optical beam ofthe first wavelength as to the first optical disc 11 and performingreading or writing of information will be described.

The disc type determination unit 22 which has determined that the typeof the optical disc 2 is the first optical disc 11 causes the opticalbeam of the first wavelength to be emitted from the first emitting unitof the first light source 231.

The optical beam of the first wavelength is split into three beams bythe first grating 239, for detection of tracking error signals and soforth, and is input to the second beam splitter 237. The optical beam ofthe first wavelength which has been input to the second beam splitter237 is reflected at a mirror face 237 a thereof, and is output to thethird beam splitter 238 side.

The optical beam of the first wavelength which is input to the thirdbeam splitter 238 is transmitted through a mirror face 238 a thereof,output to the collimator lens 242 side, where the divergent angle isconverted by the collimator lens 242 so as to be generally parallellight, provided with a predetermined phase difference at thequarter-wave plate 243, reflected off of the redirecting mirror 244, andoutput to the object lens 234 side.

The optical beam of the first wavelength which is input to the objectlens 234 is diffracted with the optical beam which has passed througheach region thereof having a predetermined diffraction order dominanttherein as described above, due to the first through third diffractionregions 251, 252, and 253 of the diffraction unit 250 provided on theincident side face thereof, and also suitably condensed on the signalrecording face of the first optical disc 11 due to the refractive powerof the lens curved face of the object lens 234. At this time, theoptical beam of the first wavelength is provided with diffractive powersuch that the optical beam passing through the regions 251, 252, and 253is in a state where spherical aberration can be reduced, and accordinglycan be suitably condensed. Note that the optical beam of the firstwavelength output from the object lens 234 is not only in a state of apredetermined divergent angle, but also is in a state of aperturerestriction.

The optical beam condensed at the first optical disc 11 is reflected atthe signal recording face, passes through the object lens 234,redirecting mirror 244, quarter-wave plate 243, and collimator lens 242,is reflected off of the mirror face 238 a of the third beam splitter238, and is output to the photosensor 245 side.

The optical beam split from the optical path of the outgoing opticalbeam reflected off of the third beam splitter 238 is condensed on thephotoreception face of the photodetector 245 by the multi-lens 246, anddetected.

Next, description will be made regarding the optical path at the time ofemitting an optical beam of the second wavelength to the second opticaldisc 12 and reading or writing information. The disc type determinationunit 22 which has determined that the type of the optical disc 2 is thesecond optical disc 12 causes the optical beam of the second wavelengthto be emitted from the second emitting unit of the second light source232.

The optical beam of the second wavelength emitted form the secondemitting unit is split into three beams by the second grating 240, fordetection of tracking error signals and so forth, and is input to thefirst beam splitter 236. The optical beam of the second wavelength whichhas been input to the first beam splitter 236 is transmitted through amirror face 236 a thereof, also transmitted through the mirror face 237a of the second beam splitter 237, and is output to the third beamsplitter 238 side.

The optical beam of the second wavelength which is input to the thirdbeam splitter 238 is transmitted through the mirror face 238 a thereof,output to the collimator lens 242 side, where the divergent angle isconverted by the collimator lens 242 so as to be in a state of diffusedlight, provided with a predetermined phase difference at thequarter-wave plate 243, reflected off of the redirecting mirror 244, andoutput to the object lens 234 side.

The optical beam of the second wavelength which is input to the objectlens 234 is diffused with the optical beam which has passed through eachregion thereof having a predetermined diffraction order dominant thereinas described above, due to the first and second diffraction regions 251and 252 of the diffraction unit 250 provided on the incident side facethereof, and also suitably condensed on the signal recording face of thesecond optical disc 12 due to the refractive power of the lens curvedface of the object lens 234. At this time, the optical beam of thesecond wavelength is provided with diffractive power such that theoptical beam passing through the first and second diffraction regions251 and 252 is in a state where spherical aberration can be reduced, andaccordingly can be suitably condensed. Also note that the diffractedlight due to the optical beam of the second wavelength which havingpassed through the third diffraction region 253 is in a state of notbeing condensed on the signal recording face of the second optical disc,i.e., suitable aperture restriction advantages can be had, due to theadvantages of the above-described flaring.

The return optical path of the optical beam reflected off of the signalrecording face of the second optical disc 12 is the same as with thecase of the above-described optical beam of the first wavelength, andaccordingly description thereof will be omitted.

Next, description will be made regarding the optical path at the time ofemitting an optical beam of the third wavelength to the third opticaldisc 13 and reading or writing information. The disc type determinationunit 22 which has determined that the type of the optical disc 2 is thethird optical disc 13 causes the optical beam of the third wavelength tobe emitted from the third emitting unit of the third light source 233.

The optical beam of the third wavelength emitted form the third emittingunit is split into three beams by the third grating 241, for detectionof tracking error signals and so forth, and is input to the first beamsplitter 236. The optical beam of the third wavelength which has beeninput to the first beam splitter 236 is reflected off of the mirror face236 a thereof, transmitted through the mirror face 237 a of the secondbeam splitter 237, and is output to the third beam splitter 238 side.

The optical beam of the third wavelength which is input to the thirdbeam splitter 238 is transmitted through the mirror face 238 a thereof,output to the collimator lens 242 side, where the divergent angle isconverted by the collimator lens 242 so as to be in a diffused lightstate, provided with a predetermined phase difference at thequarter-wave plate 243, reflected off of the redirecting mirror 244, andoutput to the object lens 234 side.

The optical beam of the third wavelength which is input to the objectlens 234 is diffused with the optical beam which has passed through eachregion thereof having a predetermined diffraction order dominant thereinas described above, due to the first diffraction region 251 of thediffraction unit 250 provided on the incident side face thereof, andalso suitably condensed on the signal recording face of the thirdoptical disc 13 due to the refractive power of the lens curved face ofthe object lens 234. At this time, the optical beam of the thirdwavelength is provided with diffractive power such that the optical beampassing through the first diffraction region 251 is in a state wherespherical aberration can be reduced, and accordingly can be suitablycondensed. Also note that the diffracted light due to the optical beamof the third wavelength which having passed through the second and thirddiffraction regions 252 and 253 is in a state of not being condensed onthe signal recording face of the third optical disc 13, i.e., suitableaperture restriction advantages can be had, due to the advantages of theabove-described flaring.

The return optical path of the optical beam reflected off of the signalrecording face of the third optical disc 13 is the same as with the caseof the above-described optical beam of the first wavelength, andaccordingly description thereof will be omitted.

Note that while a configuration has been described here wherein theoptical beam of the second and third wavelengths have the position ofthe second and/or third emitting units adjusted such that the opticalbeam of which the divergent angle is converted by the collimator lens242 and input to the object lens 234 is in a diffused state as togenerally parallel light, a configuration may be made wherein theoptical beam is input to the object lens 234 by providing an elementwhich has wavelength selectivity and converts the divergent angle, or byproviding a mechanism which drives the collimator lens 242 in theoptical axis direction in a diffused or converged state.

Also, while description has been made regarding a configuration whereinthe optical beam of the first wavelength is input to the object lens 234in a state of generally parallel light, the optical beams of the secondand third wavelengths are input to the object lens 234 in a state ofdiffused light, the present invention is not restricted to thisarrangement, and configurations may be made wherein, for example, thefirst through third wavelength optical beams are selectively input tothe object lens 234 in a state of diffused light, parallel light, orconverged light.

The optical pickup 203 to which the present invention has been appliedhas first through third emitting units for emitting optical beams offirst through third wavelengths, an object lens 234 for condensing theoptical beams of first through third wavelengths emitted from the firstthrough third emitting units into a signal recording face of an opticaldisc, and a diffraction unit 250 provided on one face of the object lens234 serving as an optical element disposed on the outgoing optical pathof the optical beams of first through third wavelengths, wherein thediffraction unit 250 has first through third diffraction regions 251,252, and 253, with the first through third diffraction regions 251, 252,and 253 being different diffraction structures ring shaped and having apredetermined depth, and the first through third diffraction structureswhereby optical beams of each wavelength are diffracted such thatdiffracted light of a predetermined diffraction order is dominant asdescribed above, and according to this configuration, optical beamscorresponding to each of three types of optical discs having differenceusage wavelengths can be appropriately condensed on the signal recordingface using a common object lens 234, thereby realizing excellentrecording and/or playing of information signals to/from the respectiveoptical discs by realizing three-wavelength compatibility with thecommon object lens 234, without necessitating a complex structure.

That is to say, the optical pickup 203 to which the present inventionhas been applied obtains optimal diffraction efficiencies anddiffraction angels for the first through third wavelength optical beamsdue to the diffraction unit 250 provided on one face within the opticalpath thereof, whereby signals can be read from and written to themultiple types of optical discs 11, 12, and 13, using the optical beamsof different wavelengths emitted from the multiple emitting unitsprovided to each of the light sources 231, 232, and 233, and alsooptical parts such as the object lens 234 and so forth can be shared,thereby reducing the number of parts, simplifying and reducing the sizeof the configuration, and realizing high production and lower costs.

Also, the optical pickup 203 to which the present invention has beenapplied is configured having the relation k1 i≧k2 i>k3 i for thepredetermined diffraction orders (k1 i, k2 i, k3 i) selected by thefirst diffraction region 251 serving as the inner ring zone, socondensing diffracted light in a state in which spherical aberration canbe reduced on the signal recording face of the corresponding opticaldiscs maximizes diffraction efficiency, which is to say in the case ofusing the third wavelength λ3, the focal distance can be prevented frombecoming too long as to the first wavelength λ1 in order to ensureoperating distance thereof, thereby preventing problems such as the lensdiameter of the object lens being large, the overall size of the opticalpickup being large, and so forth. Reducing the lens diameter of theobject lens facilitates design of the actuator, and the focal distancecan be shortened, thereby obtaining excellent aberration properties.Accordingly, information signals can be suitably recorded to and/orplayed from respective optical discs with excellent compatibility beingrealized, the configuration can be further simplified and the sizereduced, realizing high productivity and low costs.

Also, the optical pickup 203 to which the present invention has beenapplied is configured such that, of the diffraction orders (k1 i, k2 i,k3 i) selected by the first diffraction region 251 serving as the innerring zone, k1 i and k3 i are (−2, −3), (−1, −2), (−1, −3), (0, −2), (0,−3), (1, −2), (1, −3), (2, −1), (2, −2), (2, −3), (3, 0), (3, −1), (3,−2), or (3, −3), thereby preventing problems such as the lens diameterof the object lens being large, the overall size of the device beinglarge, and so forth, with the operating distance and focal distance foreach wavelength being in a suitable state, and additionally, the groovesare prevented from becoming too deep, whereby the manufacturing processcan be simplified, and also deterioration of forming precision can beprevented. Accordingly, information signals can be suitably recorded toand/or played from respective optical discs with excellent compatibilitybeing realized, the configuration can be simplified and the size reducedwhile facilitating manufacturing, realizing high productivity and lowcosts.

Also, the optical pickup 203 to which the present invention has beenapplied is configured such that the first diffraction region 251 servingas the inner ring zone which provides the three wavelengths withpredetermined diffractive power and needs high diffraction efficiency,has formed a stepped diffraction structure, thereby suppressing theamount of diffracted light of unwanted light, preventing deteriorationof jittering and the like due to unwanted light being received at thephotosensor, and also, even in cases of a certain amount of diffractedlight of unwanted light occurring, unwanted light being received at thephotosensor at the time of focusing leading to deterioration ofjittering and the like can be prevented by making the diffraction orderof the unwanted light to be a deviated order with great diffractionangle difference, that is other than an adjacent diffraction order ofthe focus light.

Also, the optical pickup 203 to which the present invention has beenapplied is configured having the outer ring zone formed integrally onone face of the object lens 234 and also provided on the outermost sidethereof, formed as a blazed form diffraction structure at the thirddiffraction region 235, which is an advantageous structure in the caseof forming a diffraction structure at portions having an extremely steeplens curved surface, such as with a three-wavelength-compatible lens,whereby manufacturing can be facilitated and deterioration in formingprecision can be prevented.

Also, the optical pickup 203 to which the present invention has beenapplied is configured such that the diffraction orders (k1 i, k2 i, k3i) of light selected by the first diffraction region 251 are (1, −1,−2), (0, −1, −2), (1, −2, −3) or (0, −2, −3), and the diffractionstructure is configured in a staircase form, so adverse affects ofunwanted light can be suppressed, the operating distance and focaldistance for each wavelength can be made to be in a suitable state andthe lens diameter of the object lens and the size of the device can beprevented from being large, and additionally, the grooves are preventedfrom becoming too deep, whereby the manufacturing process can besimplified, and also deterioration of forming precision can beprevented. Accordingly, information signals can be suitably recorded toand/or played from respective optical discs with excellent compatibilitybeing realized, the configuration can be simplified and the size reducedwhile facilitating manufacturing, realizing high productivity and lowcosts.

Also, with the optical pickup 203 to which the present invention hasbeen applied, in addition to the diffraction order selected by the innerring zone, the diffraction orders (k1 m, k2 m) of light selected by thesecond diffraction region 252 serving as the middle ring zone are (+1,+1), (−1, −1), (0, +2), (0, −2), (0, +1), (0, −1), (+1, 0), (−1, 0),(+1, −1), or (−1, +1), and the diffraction structure is configured as astaircase form or non-cyclical form, whereby the functions of the innerring zone and middle ring zone can be each sufficiently manifested.Accordingly, information signals can be suitably recorded to and/orplayed from respective optical discs with excellent compatibility beingrealized, the configuration can be simplified and the size reduced whilefacilitating manufacturing, realizing high productivity and low costs.

Also, with the optical pickup 203 to which the present invention hasbeen applied, in addition to the diffraction order selected by the innerring zone, the diffraction orders (k1 m, k2 m) of light selected by thesecond diffraction region 252 serving as the middle ring zone are (+3,+2), (−3, −2), (+2, +1), (−2, −1), (+1, −1), or (−1, −1), and thediffraction structure is configured as a blazed form or non-cyclicalform, whereby the functions of the inner ring zone and middle ring zonecan be each sufficiently manifested. Accordingly, information signalscan be suitably recorded to and/or played from respective optical discswith excellent compatibility being realized, the configuration can besimplified and the size reduced while facilitating manufacturing,realizing high productivity and low costs.

Also, with the optical pickup 203 to which the present invention hasbeen applied, at the time of input of the condensing optical device suchas the object lens 234 or the like, the optical beam of the firstwavelength is generally parallel light and the optical beams of thesecond and third wavelengths are input as diffused light, and due tothis configuration, the optical beams passing through the firstdiffraction region 251 serving as the inner ring zone can be suitablycondensed on the signal recording face of the corresponding optical discin a state of high diffractive efficiency and even further reducedspherical aberration, and also the advantages of flaring can be had atthe second and third diffraction regions serving as the middle ring zoneand outer ring zone, high efficiency and reduced spherical aberrationcan be realized for optical beams of a predetermined wavelength whilethe quantity of light input to the corresponding signal recording facecan be reduced for optical beams of wavelengths regarding whichcondensing is undesirable, and further, the freedom of diffraction orderselection can be improved and simplification of configuration and soforth realized.

Also, the optical pickup 203 to which the present invention has beenapplied can share the object lens 234 between the three wavelengths,thereby preventing trouble such as reduction of sensitivity of theactuator due to increased weight of moving parts, and the attachmentangle of the actuator to lens holder being unsuitable, and so forth.Also, the optical pickup 203 to which the present invention has beenapplied can sufficiently reduce spherical aberration which isproblematic in the case of sharing the object lens 234 between the threewavelengths, due to the diffraction unit 250 provided on one face of theoptical element (object lens 234, diffraction optical element 235B), soproblems such as positioning of diffraction units in the event thatdiffraction units are provided on multiple faces to reduce sphericalaberration as with the related art, and deterioration of diffractionefficiency due to providing of the multiple diffraction units and soforth, can be prevented, which realizes simplification of the assemblyprocess and improved usage efficiency of light. Also, with the opticalpickup 203 to which the present invention has been applied, aconfiguration such as described above, wherein the diffraction unit 250is provided on one face of the optical element enables a configurationhaving the object lens 234 integrally formed with the diffraction unit250, realizes further simplification of the configuration, reduction inweight of moving parts of the actuator, simplification of the assemblyprocess, and improved usage efficiency of light.

Further, as shown in FIGS. 58A and 58B described above, with the opticalpickup 203 to which the present invention has been applied, thediffraction unit 250 provided on one face of the object lens 234 ordiffraction optical element 235B not only realizes three-wavelengthcompatibility, but also enables aperture restriction by numericalaperture to be performed corresponding to the three types of opticaldiscs and optical beams of the three wavelengths, thereby doing awaywith the need for aperture restriction filters or the like which havebeen necessary with the related art, and also adjustment in thepositioning thereof, realizing further simplification of theconfiguration, reduction in size, and reduction in costs.

Also, the optical pickup 203 has been described above with aconfiguration wherein the first emitting unit is provided at the firstlight source 231, the second emitting unit is provided at the secondlight source 232, and the third emitting unit is provided at the thirdlight source 233, the invention is not restricted to this, and anarrangement may be made wherein two emitting units of the first throughthird emitting units are disposed at one light source and the remainingemitting unit is disposed at another light source, for example.

Next, description will be made regarding an optical pickup 260 shown inFIG. 59 including a light source having a first emitting unit, and alight source having second and third emitting units. Note that portionsin the following description which are the same as with the opticalpickup 203 will be denoted with the same reference numerals, anddescription thereof will be omitted.

As shown in FIG. 59, the optical pickup 260 to which the presentinvention has been applied includes a first light source 261 having afirst emitting unit for emitting an optical beam of a first wavelength,a second light source 262 having a second emitting unit for emitting anoptical beam of a second wavelength and a third emitting unit foremitting an optical beam of a third wavelength, and an object lens 234serving as a condensing optical device for condensing optical beamsemitted from the first through third emitting units onto the signalrecording face of an optical disc 2. Also, with the optical pickup 260described here as well, a configuration may be made wherein a condensingoptical device configured of the object lens 234B and the diffractionoptical element 235B having the diffraction unit 250 such as shown inFIG. 58B is provided instead of the object lens 234 having thediffraction unit 250 described here.

Also, the optical pickup 260 includes a beam splitter 263 serving as anoptical path synthesizing unit for synthesizing the optical paths of theoptical beam of the first wavelength that has been emitted from thefirst emitting unit of the first light source 261 and the optical beamsof the second and third wavelengths that have been emitted from thesecond and third emitting units of the second light source 262, and abeam splitter 264 serving the same function as the above third beamsplitter 238.

Further, the optical pickup 260 has a first grating 239, and a grating265 with wavelength dependency, provided between the second light sourceunit 262 and the beam splitter 263, for diffracting the optical beams ofthe second and third wavelengths that have been emitted from the secondand third emitting units into three beams, for detection of trackingerror signals and so forth.

Also, the optical pickup 260 has a collimator lens 242, quarter-waveplate 243, redirecting mirror 244, photosensor 245, and multi-lens 246,and also a collimator lens driving unit 266 for driving the collimatorlens 242 in the direction of the optical axis. The collimator lensdriving unit 266 can adjust the divergent angle of optical beams passingthrough the collimator lens 242 as described above by driving thecollimator lens 242 in the direction of the optical axis, whereby notonly can spherical aberration be reduced by inputting each optical beamto the object lens 234 in a predetermined state enabling theabove-described flaring, but in the event that the mounted optical discis a so-called multi-layer optical disc having multiple signal recordingfaces, recording and/or playing to/from each of the signal recordingfaces is enabled.

With the optical pickup 260 configured as described above, the functionsof each of the optical parts is the same as with the optical pickup 203except for those mentioned above, and the optical paths of the opticalbeams of the first through third wavelengths emitted from the firstthrough third emitting units are the same as with the optical pickup 203except for the above-mentioned, i.e., following synthesizing of theoptical paths of the optical beams of each wavelength by the beamsplitter 264, so detailed description thereof will be omitted.

The optical pickup 260 to which the present invention has been appliedhas first through third emitting units for emitting optical beams offirst through third wavelengths, an object lens 234 for condensing theoptical beams of first through third wavelengths emitted from the firstthrough third emitting units into a signal recording face of an opticaldisc, and a diffraction unit 250 provided on one face of the object lens234 serving as an optical element disposed on the outgoing optical pathof the optical beams of first through third wavelengths, wherein thediffraction unit 250 has first through third diffraction regions 251,252, and 253, with the first through third diffraction regions 251, 252,and 253 being different diffraction structures ring shaped and having apredetermined depth, and the first through third diffraction structureswhereby optical beams of each wavelength are diffracted such thatdiffracted light of a predetermined diffraction order is dominant asdescribed above, and according to this configuration, optical beamscorresponding to each of three types of optical discs having differentusage wavelengths can be appropriately condensed on the signal recordingface using the single shared object lens 234, thereby realizingexcellent recording and/or playing of information signals to/from therespective optical discs by realizing three-wavelength compatibilitywith the common object lens 234, without necessitating a complexstructure. The optical pickup 260 also has the other advantages of theabove-described optical pickup 203, as well.

Further, the optical pickup 260 is configured such that the second andthird emitting units are positioned at a common light source 262,thereby realizing further simplification of configuration and reductionin size. Note that in the same way, with the optical pickup to which thepresent invention has been applied, the first through third emittingunits may be positioned at a light source at generally the sameposition, thereby realizing further simplification of configuration andreduction in size with such a configuration.

The optical disc device 1 to which the present invention has beenapplied has a driving unit for holding and rotationally driving anoptical disc arbitrarily selected from the first through third opticaldiscs, and an optical pickup for performing recording and/or playing ofinformation signals from/to the optical disc being rotationally drivenby the driving unit by selectively irradiating one of multiple opticalbeams of different wavelengths corresponding to the optical disc, and byusing the above-described optical pickups 203 or 260 as the opticalpickup, optical beams corresponding to each of three types of opticaldiscs having different usage wavelengths can be appropriately condensedon the signal recording face due to the diffraction unit provided on oneface of the optical element on the optical path of the optical beams ofthe first through third wavelengths, using a single shared object lens234, thereby realizing excellent recording and/or playing properties byrealizing three-wavelength compatibility with the common object lens234, while enabling simplification of the structure and reduction insize, without necessitating a complex structure.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. An optical pickup comprising: a first emitting unit configured to emit an optical beam of a first wavelength corresponding to a first optical disc having a first transmissive layer; a second emitting unit configured to emit an optical beam of a second wavelength which is longer than said first wavelength, corresponding to a second optical disc which is different from said first optical disc and has a second transmissive layer thicker than said first transmissive layer; a third emitting unit configured to emit an optical beam of a third wavelength which is longer than said second wavelength, corresponding to a third optical disc which is different from said first and second optical discs and has a third transmissive layer thicker than said second transmissive layer; an object lens configured to condense optical beams emitted from said first through third emitting units onto a signal recording face of an optical disc; and a diffraction unit provided on one face of an optical element or said object lens positioned on the optical path of said optical beams of said first through third wavelengths; wherein said diffraction unit includes a generally circular first diffraction region provided on the innermost perimeter, a ring zone shaped second diffraction region provided on the outer side of said first diffraction region, and a ring zone shaped third diffraction region or aspheric continuous region, provided on the outer side of said second diffraction region; wherein said first diffraction region has a first diffraction structure formed in a generally circular shape and having a predetermined depth, where the spherical aberration polarity given to the diffracted light of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens, is of opposite polarity as to the spherical aberration polarity given to the diffracted light of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens, and as to the spherical aberration polarity given to the diffracted light of said of said optical beam of the third wavelength which passes therethrough and is condensed on the signal recording face of said third optical disc via said object lens; and wherein said second diffraction region has a second diffraction structure which is different from said first diffraction structure formed in a ring zone shape and having a predetermined depth, which emits diffracted light of an order of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens, emits diffracted light of an order of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens, and emits diffracted light such that diffracted light of an order other than the order of said optical beam of the third wavelength which passes therethrough and is condensed on the signal recording face of said third optical disc via said object lens is dominant; and wherein said third diffraction region collects said optical beam of the first wavelength which passes therethrough on the signal recording face of said first optical disc via said object lens, does not collect said optical beam of the second wavelength which passes therethrough on the signal recording face of said second optical disc via said object lens, and does not collect said optical beam of the third wavelength which passes therethrough on the signal recording face of said third optical disc via said object lens.
 2. The optical pickup according to claim 1, wherein said first diffraction region emits diffracted light such that diffracted light of an order k1 i of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders, emits diffracted light such that diffracted light of an order k2 i of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders, and emits diffracted light such that diffracted light of an order k3 i of said optical beam of the third wavelength which passes therethrough and is condensed on the signal recording face of said third optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders; and wherein said k1 i and said k2 i are of the opposite sign, and said k2 i and said k3 i are of the same sign.
 3. The optical pickup according to claim 2, wherein said first diffraction region emits diffracted light so as to have the relation of k1 i>k2 i>k3 i wherein an order diffracting toward the optical axis direction of the input optical beam is a positive order.
 4. The optical pickup according to claim 3, wherein the thickness of said first transmissive layer is around 0.1 mm, the thickness of said second transmissive layer is around 0.6 mm, the thickness of said third transmissive layer is around 1.1 mm, said first wavelength is around 405 nm, said second wavelength is around 655 nm, said third wavelength is around 785 nm; and wherein said k1 i and k3 i are (1, −2), (1, −3), (2, −1), (2, −2), (2, −3), (3, −1), (3, −2), or (3, −3), respectively.
 5. The optical pickup according to claim 3, wherein said first diffraction region has a staircase form diffraction structure formed, in which a staircase structure having a plurality of steps is continuously formed in the radial direction of the ring zone; and wherein said second diffraction region has a staircase form diffraction structure formed, in which a staircase form having a plurality of steps is continuously formed in the radial direction of the ring zone, or a blazed form; and wherein said third diffraction region has a diffraction structure formed of a blazed form.
 6. The optical pickup according to claim 3, wherein said first diffraction region has a non-cyclical diffraction structure formed, in which a non-cyclical structure is formed in the radial direction of the ring zone; and wherein said second diffraction region has a non-cyclical diffraction structure formed, in which a non-cyclical structure is formed in the radial direction of the ring zone, or a blazed form; and wherein said third diffraction region has a diffraction structure formed of a blazed form.
 7. The optical pickup according to claim 3, wherein the thickness of said first transmissive layer is around 0.1 mm, the thickness of said second transmissive layer is around 0.6 mm, the thickness of said third transmissive layer is around 1.1 mm, said first wavelength is around 405 nm, said second wavelength is around 655 nm, said third wavelength is around 785 nm; and wherein said k1 i, k2 i, and k3 i are (1, −1, −2) or (1, −2, −3), respectively.
 8. The optical pickup according to claim 7, wherein said second diffraction region has a staircase form diffraction structure formed, in which a staircase form having a plurality of steps is continuously formed in the radial direction of the ring zone, or a non-cyclical diffraction structure formed, in which a non-cyclical structure is formed in the radial direction of the ring zone; and wherein said second diffraction region emits diffracted light such that diffracted light of an order k1 m of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders, and emits diffracted light such that diffracted light of an order k2 m of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders; and wherein said k1 m and k2 m are (+1, +1), (−1, −1), (0, +2), (0, −2), (0, +1), (0, −1), (+1, 0), (−1, 0), (+1, −1), or (−1, +1), respectively.
 9. The optical pickup according to claim 7, wherein said second diffraction region has a blazed form diffraction structure formed; and wherein said second diffraction region emits diffracted light such that diffracted light of an order k1 m of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders, and emits diffracted light such that diffracted light of an order k2 m of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders; and wherein said k1 m and k2 m are (+3, +2), (−3, 31 2), (+2, +1), (−2, −1), (+1, +1), or (−1, −1), respectively.
 10. The optical pickup according to claim 3, wherein at the time of input to the input side face of the closer-disposed element, of said object lens or the optical element to which said diffraction unit has been provided, to said first through third emitting units, the optical beam of the first wavelength is input as generally parallel light, and the optical beams of the second and third wavelengths as diffused light.
 11. The optical pickup according to claim 1, wherein said first through third diffraction regions emit diffracted light such that diffracted light of said optical beams of the first through third wavelengths passing therethrough of an order other than zero order is dominant.
 12. The optical pickup according to claim 1, wherein said first through third diffraction regions each have a staircase form diffraction structure formed, in which a staircase form having a plurality of steps is continuously formed in the radial direction of the ring zone.
 13. The optical pickup according to claim 1, wherein said first and second diffraction regions each have a staircase form diffraction structure formed, in which a staircase form having a plurality of steps is continuously formed in the radial direction of the ring zone; and wherein said third diffraction region has a blazed form diffraction structure formed.
 14. The optical pickup according to claim 1, further comprising a divergent angle converting element configured to convert the divergent angle of the optical beams emitted from said first through third emitting units; wherein said divergent angle converting element converts the divergent angle of said optical beams of the first through third wavelengths, and at the time of input to the input side face of the closer-disposed element of said object lens or the optical element to which said diffraction unit has been provided to said first through third emitting units, the optical beams of the first and second wavelengths are input as generally parallel light, and the optical beam of the third wavelength as converged light or diffused light.
 15. The optical pickup according to claim 2, wherein the thickness of said first transmissive layer is around 0.1 mm, the thickness of said second transmissive layer is around 0.6 mm, the thickness of said third transmissive layer is around 1.1 mm, said first wavelength is around 405 nm, said second wavelength is around 655 nm, said third wavelength is around 785 nm; and wherein said k1 i, k2 i, and k3 i are (+1, −1, −2), (−1, +1, +2), (+1, −, −2, −3), (−1, +2, +3), (+2, −1, −2), (−2, +1, +2), (+2, −2, −3), or (−2, +2, +3), respectively.
 16. The optical pickup according to claim 1, wherein said first through third diffraction regions are formed to a size such that said optical beam of the first wavelength passing therethrough becomes a corresponding first numerical aperture; and wherein said first and second diffraction regions are formed to a size such that said optical beam of the second wavelength passing therethrough becomes a corresponding second numerical aperture; and wherein said first diffraction region is formed to a size such that said optical beam of the third wavelength passing therethrough becomes a corresponding third numerical aperture.
 17. An optical disc device comprising: driving means configured to hold and rotationally drive an optical disc optionally selected from at least a first optical disc having a first transmissive layer, a second optical disc of a different type from said first optical disc and having a second transmissive layer thicker than said first transmissive layer, and a third optical disc of a different type from said first and second optical discs and having a third transmissive layer thicker than said second transmissive layer; and an optical pickup configured to selectively irradiate multiple optical beams of different wavelengths to an optical disc rotationally driven by said driving means, so as to record and/or play information signals; said optical pickup including a first emitting unit configured to emit an optical beam of a first wavelength corresponding to said first optical disc, a second emitting unit configured to emit an optical beam of a second wavelength which is longer than said first wavelength, corresponding to said second optical disc, a third emitting unit configured to emit an optical beam of a third wavelength which is longer than said second wavelength, corresponding to said third optical disc, an object lens configured to condense optical beams emitted from said first through third emitting units onto a signal recording face of an optical disc, and a diffraction unit provided on one face of an optical element or said object lens, positioned on the optical path of said optical beams of the first through third wavelengths; wherein said diffraction unit includes a generally circular first diffraction region provided on the innermost perimeter, a ring zone shaped second diffraction region provided on the outer side of said first diffraction region, and a ring zone shaped third diffraction region or aspheric continuous region provided on the outer side of said second diffraction region; wherein with said first diffraction region, the spherical aberration polarity given to the diffracted light of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens, is of opposite polarity as to the spherical aberration polarity given to the diffracted light of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens, and as to the spherical aberration polarity given to the diffracted light of said of said optical beam of the third wavelength which passes therethrough and is condensed on the signal recording face of said third optical disc via said object lens; and wherein said second diffraction region has a second diffraction structure which is different from said first diffraction structure formed in a ring zone shape and having a predetermined depth, which emits diffracted light of an order of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens, emits diffracted light of an order of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens, and emits diffracted light such that diffracted light of an order other than the order of said optical beam of the third wavelength which passes therethrough and is condensed on the signal recording face of said third optical disc via said object lens is dominant; and wherein said third diffraction region collects said optical beam of the first wavelength which passes therethrough on the signal recording face of said first optical disc via said object lens, does not collect said optical beam of the second wavelength which passes therethrough on the signal recording face of said second optical disc via said object lens, and does not collect said optical beam of the third wavelength which passes therethrough on the signal recording face of said third optical disc via said object lens.
 18. An object lens used with an optical pickup configured to irradiate optical beams on at least a first optical disc having a first transmissive layer, a second optical disc of a different type from said first optical disc and having a second transmissive layer thicker than said first transmissive layer, and a third optical disc of a different type from said first and second optical discs and having a third transmissive layer thicker than said second transmissive layer; and so as to record and/or play information signals, with said object lens condensing an optical beam of a first wavelength corresponding to said first optical disc, an optical beam of a second wavelength which is longer than said first wavelength, corresponding to said second optical disc, and an optical beam of a third wavelength which is longer than said second wavelength, corresponding to said third optical disc, onto a signal recording face of a corresponding optical disc, said object lens comprising: a diffraction unit provided on the input side face or output side face; wherein said diffraction unit includes a generally circular first diffraction region provided on the innermost perimeter, a ring zone shaped second diffraction region provided on the outer side of said first diffraction region, and a ring zone shaped third diffraction region or aspheric continuous region provided on the outer side of said second diffraction region; wherein with said first diffraction region, the spherical aberration polarity given to the diffracted light of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens, is of opposite polarity as to the spherical aberration polarity given to the diffracted light of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens, and as to the spherical aberration polarity given to the diffracted light of said of said optical beam of the third wavelength which passes therethrough and is condensed on the signal recording face of said third optical disc via said object lens; and wherein said second diffraction region has a second diffraction structure which is different from said first diffraction structure formed in a ring zone shape and having a predetermined depth, which emits diffracted light of an order of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens, emits diffracted light of an order of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens, and emits diffracted light such that diffracted light of an order other than the order of said optical beam of the third wavelength which passes therethrough and is condensed on the signal recording face of said third optical disc via said object lens is dominant; and wherein said third diffraction region collects said optical beam of the first wavelength which passes therethrough on the signal recording face of said first optical disc via said object lens, does not collect said optical beam of the second wavelength which passes therethrough on the signal recording face of said second optical disc via said object lens, and does not collect said optical beam of the third wavelength which passes therethrough on the signal recording face of said third optical disc via said object lens.
 19. The optical lens according to claim 18, wherein said first diffraction region emits diffracted light such that diffracted light of an order k1 i of said optical beam of the first wavelength which passes therethrough and is condensed on the signal recording face of said first optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders, emits diffracted light such that diffracted light of an order k2 i of said optical beam of the second wavelength which passes therethrough and is condensed on the signal recording face of said second optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders, and emits diffracted light such that diffracted light of an order k3 i of said optical beam of the third wavelength which passes therethrough and is condensed on the signal recording face of said third optical disc via said object lens has maximum diffraction efficiency as to diffracted light of the other orders; and wherein said k1 i and said k2 i are of the opposite sign, and said k2 i and said k3 i are of the same sign.
 20. The optical lens according to claim 19, wherein said first diffraction region emits diffracted light so as to have the relation of k1 i>k2 i>k3 i wherein an order diffracting toward the optical axis direction of the input optical beam is a positive order.
 21. The optical lens according to claim 20, wherein said first diffraction region has a staircase form diffraction structure formed, in which a staircase structure having a plurality of steps is continuously formed in the radial direction of the ring zone; and wherein said second diffraction region has a staircase form diffraction structure formed, in which a staircase form having a plurality of steps is continuously formed in the radial direction of the ring zone, or a blazed form; and wherein said third diffraction region has a diffraction structure formed of a blazed form.
 22. The optical lens according to claim 20, wherein said first diffraction region has a non-cyclical diffraction structure formed, in which a non-cyclical structure is formed in the radial direction of the ring zone; and wherein said second diffraction region has a non-cyclical diffraction structure formed, in which a non-cyclical structure is formed in the radial direction of the ring zone, or a blazed form; and wherein said third diffraction region has a diffraction structure formed of a blazed form.
 23. The optical lens according to claim 20, wherein at the time of input to the input side face of the closer-disposed element, of said object lens or the optical element to which said diffraction unit has been provided, to said first through third emitting units, the optical beam of the first wavelength is input as generally parallel light, and the optical beams of the second and third wavelengths as diffused light.
 24. The optical lens according to claim 18, wherein said first through third diffraction regions emit diffracted light such that diffracted light of said optical beams of the first through third wavelengths passing therethrough of an order other than zero order is dominant.
 25. The optical lens according to claim 18, wherein said first through third diffraction regions each have a staircase form diffraction structure formed, in which a staircase form having a plurality of steps is continuously formed in the radial direction of the ring zone.
 26. The optical lens according to claim 18, wherein said first and second diffraction regions each have a staircase form diffraction structure formed, in which a staircase form having a plurality of steps is continuously formed in the radial direction of the ring zone; and wherein said third diffraction region has a blazed form diffraction structure formed.
 27. The optical lens according to claim 18, further comprising a divergent angle converting element configured to convert the divergent angle of the optical beams emitted from said first through third emitting units; wherein said divergent angle converting element converts the divergent angle of said optical beams of the first through third wavelengths, and at the time of input to the input side face of the closer-disposed element of said object lens or the optical element to which said diffraction unit has been provided to said first through third emitting units, the optical beams of the first and second wavelengths are input as generally parallel light, and the optical beam of the third wavelength as converged light or diffused light.
 28. The optical lens according to claim 18, wherein said first through third diffraction regions are formed to a size such that said optical beam of the first wavelength passing therethrough becomes a corresponding first numerical aperture; and wherein said first and second diffraction regions are formed to a size such that said optical beam of the second wavelength passing therethrough becomes a corresponding second numerical aperture; and wherein said first diffraction region is formed to a size such that said optical beam of the third wavelength passing therethrough becomes a corresponding third numerical aperture. 